This refactors all instances of bitwise-ANDs with `~MOD_MASK'. There is
a handful of more complex instances I left unchanged (in cube.c, midend.c,
and twiddle.c), since those AND with `~MOD_MASK | MOD_NUM_KEYPAD' or
similar. I don't think it's worth writing a macro for those cases.

Also document this new macro's usage in devel.but.

In Jigsaw Solo, block boundaries aren't convex, so it's possible for
one of them to have an inward corner. If that corner is in the top
left of a selectable cell, and you right-click that cell to display
the pencil-mode 'cursor' in the form of a triangle in the top left,
then the cursor was accidentally drawn on top of the block boundary,
where it ought to be underneath it.

For example, in game id 5j:d1d4_4c3_1d2d,bb_baaa_dca_baaba, right-
clicking in the bottom right square of the grid demonstrates the
problem.

Jonas Kölker fixed this for Keen in 2015, in commit 6482ed0e3c886af.
This is the identical fix, in Solo's very similar-looking drawing
routine. I feel embarrassed to have taken eight years to get round to
it!

If you're using the mouse to change pencil marks, you have to
right-click to pencil-highlight a square, then press a number or
letter key to add or remove a highlight. That causes the highlight to
vanish again. So adding or removing multiple pencil marks requires a
right-click + keypress per mark.

Chris's Android port reversed that decision, making the pencil
highlight persist so that you could 'click' just once and then press
multiple pencil keys. That makes it easier to add lots of highlights,
but harder to just remove a single one (click + press + click to
remove the highlight), unless you don't mind keeping the highlight
around afterwards cluttering up your view.

In other words, this is just the sort of thing users might reasonably
disagree on. So now we have an organised preferences system, we can
let them disagree, and each configure it whichever way they like!

This only affects mouse-based play. The keyboard cursor has _always_
worked this way, because it doesn't disappear at all; its behaviour is
unchanged, and independent of the new preference.

This adds an extra parameter to move_cursor() that's an optional pointer
to a bool indicating whether the cursor is visible.  This allows for
centralising the common idiom of having the keyboard cursor become
visible when a cursor key is pressed.  Consistently with the vast
majority of existing puzzles, the cursor moves even if it was invisible
before, and becomes visible even if it can't move.

The function now also returns one of the special constants that can be
returned by interpret_move(), so that the caller can correctly return
MOVE_UI_UPDATE or MOVE_NO_EFFECT without needing to carefully check for
changes itself.

Callers are updated only to the extent that they all pass NULL as the
new argument.  Most of them could now be substantially simplified.

All the other constants named UI_* are special key names that can be
passed to midend_process_key(), but UI_UPDATE is a special return value
from the back-end interpret_move() function instead.  This renaming
makes the distinction clear and provides a naming convention for future
special return values from interpret_move().

Thanks to Jeremy Stephens for reporting them.

These are similar to the existing pair configure() and custom_params()
in that get_prefs() returns an array of config_item describing a set
of dialog-box controls to present to the user, and set_prefs()
receives the same array with answers filled in and implements the
answers. But where configure() and custom_params() operate on a
game_params structure, the new pair operate on a game_ui, and are
intended to permit GUI configuration of all the settings I just moved
into that structure.

However, nothing actually _calls_ these routines yet. All I've done in
this commit is to add them to 'struct game' and implement them for the
functions that need them.

Also, config_item has new fields, permitting each config option to
define a machine-readable identifying keyword as well as the
user-facing description. For options of type C_CHOICES, each choice
also has a keyword. These keyword fields are only defined at all by
the new get_prefs() function - they're left uninitialised in existing
uses of the dialog system. The idea is to use them when writing out
the user's preferences into a configuration file on disk, although I
haven't actually done any of that work in this commit.

I'm about to move some of the bodgy getenv-based options so that they
become fields in game_ui. So these functions, which could previously
access those options directly via getenv, will now need to be given a
game_ui where they can look them up.

This is preparing to separate out the auxiliary functionality, and
perhaps leave space for making more of it in future.

The previous name 'edsf' was too vague: the 'e' stood for 'extended',
and didn't say anything about _how_ it was extended. It's now called a
'flip dsf', since it tracks whether elements in the same class are
flipped relative to each other. More importantly, clients that are
going to use the flip tracking must say so when they allocate the dsf.

And Keen's need to track the minimal element of an equivalence class
is going to become a non-default feature, so there needs to be a new
kind of dsf that specially tracks those, and Keen will have to call it.

While I'm here, I've renamed the three dsf creation functions so that
they start with 'dsf_' like all the rest of the dsf API.