Adding an association with an out-of-bounds square (i.e. by pressing Return
with a dot selected, and then moving the cursor so the `opposite' arrow was
off the screen) would cause space_opposite_dot() to return NULL, in turn
causing ok_to_add_assoc_with_opposite_internal() to return false, failing
the assertion.

This assertion appears to have been introduced in 68363231.

The Rockbox frontend allows games to be displayed in a "zoomed-in"
state targets with small displays. Currently we use a modal interface
-- a "viewing" mode in which the cursor keys are used to pan around
the rendered bitmap; and an "interaction" mode that actually sends
keys to the game.

This commit adds a midend_get_cursor_location() function to allow the
frontend to retrieve the backend's cursor location or other "region of
interest" -- such as the player location in Cube or Inertia.

With this information, the Rockbox frontend can now intelligently
follow the cursor around in the zoomed-in state, eliminating the need
for a modal interface.

If you drag an arrow on to a square which is already filled in as part
of a completed region, or whose counterpart is filled in, or whose
counterpart is actually a dot, then the game can't actually place a
double arrow. Previously, it didn't find that out until execute_move
time, at which point it was too late to prevent a no-op action from
being placed on the undo chain.

Now we do those checks in interpret_move, before generating the move
string that tries to place the double arrow in the first place. So
execute_move can now enforce by assertion that arrow-placement moves
it gets are valid.

Another thing I spotted while trawling the whole source base was that
a couple of games had omitted 'static' on a lot of their internal
functions. Checking with nm, there turned out to be quite a few more
than I'd spotted by eye, so this should fix them all.

Also added one missing 'const', on the lookup table nbits[] in Tracks.

This is the main bulk of this boolification work, but although it's
making the largest actual change, it should also be the least
disruptive to anyone interacting with this code base downstream of me,
because it doesn't modify any interface between modules: all the
inter-module APIs were updated one by one in the previous commits.
This just cleans up the code within each individual source file to use
bool in place of int where I think that makes things clearer.

This commit removes the old #defines of TRUE and FALSE from puzzles.h,
and does a mechanical search-and-replace throughout the code to
replace them with the C99 standard lowercase spellings.

encode_params, validate_params and new_desc now take a bool parameter;
fetch_preset, can_format_as_text_now and timing_state all return bool;
and the data fields is_timed, wants_statusbar and can_* are all bool.
All of those were previously typed as int, but semantically boolean.

This commit changes the API declarations in puzzles.h, updates all the
games to match (including the unfinisheds), and updates the developer
docs as well.

This function gives the front end a way to find out what keys the back
end requires; and as such it is mostly useful for ports without a
keyboard. It is based on changes originally found in Chris Boyle's
Android port, though some modifications were needed to make it more
flexible.

I've also added that warning option and -Werror to the build script,
so that I'll find out if I break this property in future.

They're never dynamically allocated, and are almost always string
literals, so const is more appropriate.