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().

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.

Now that the dsf knows its own size internally, there's no need to
tell it again when one is copied or reinitialised.

This makes dsf_init much more about *re*initialising a dsf, since now
dsfs are always allocated using a function that will initialise them
anyway. So I think it deserves a rename.

In this commit, 'DSF' is simply a typedef for 'int', so that the new
declaration form 'DSF *' translates to the same type 'int *' that dsfs
have always had. So all we're doing here is mechanically changing type
declarations throughout the code.

All remaining cases where a dsf was allocated via snewn(foo, int) are
removed by this change.

No functional change: currently, this just wraps the previous sfree
call.

The majority of back-ends define encode_ui() to return NULL and
decode_ui() to do nothing.  This commit allows them to instead specify
the relevant function pointers as NULL, in which case the mid-end won't
try to call them.

I'm planning to add a parameter to decode_ui(), and if I'm going to have
to touch every back-end's version of decode_ui(), I may as well ensure
that most of them never need to be touched again.  And obviously
encode_ui() should go the same way for symmetry.

This fixes a build failure introduced by commit 2e48ce132e011e8
yesterday.

When I saw that commit I expected the most likely problem would be in
the NestedVM build, which is currently the thing with the most most
out-of-date C implementation. And indeed the NestedVM toolchain
doesn't have <tgmath.h> - but much more surprisingly, our _Windows_
builds failed too, with a compile error inside <tgmath.h> itself!

I haven't looked closely into the problem yet. Our Windows builds are
done with clang, which comes with its own <tgmath.h> superseding the
standard Windows one. So you'd _hope_ that clang could make sense of
its own header! But perhaps the problem is that this is an unusual
compile mode and hasn't been tested.

My fix is to simply add a cmake check for <tgmath.h> - which doesn't
just check the file's existence, it actually tries compiling a file
that #includes it, so it will detect 'file exists but is mysteriously
broken' just as easily as 'not there at all'. So this makes the builds
start working again, precisely on Ben's theory of opportunistically
using <tgmath.h> where possible and falling back to <math.h>
otherwise.

It looks ugly, though! I'm half tempted to make a new header file
whose job is to include a standard set of system headers, just so that
that nasty #ifdef doesn't have to sit at the top of almost all the
source files. But for the moment this at least gets the build working
again.