Not all platforms support printing floats, this is a more
portable (if uglier) way.

"swap_regions()" is first of all more descriptive, but more
importantly, it doesn't break builds on Rockbox!

The 'decode' flag to obfuscate_bitmap and the 'wrap' flag to
move_cursor are the only ones affected here.

The previous version attempted to free the first element multiple times.

Checking with the standards, I think this is legal C99, but not legal
C89 - and we are compiling in C89 mode. Why _every_ version of gcc
didn't object, given all the warning and pedantry options, I'm not
sure, but one did, so I should fix it.

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'm about to want to use it outside the GTK front end.

I went through all the char * parameters and return values I could see
in puzzles.h by eye and spotted ones that surely ought to have been
const all along.

This allows me to use different types for the mutable, dynamically
allocated string value in a C_STRING control and the fixed constant
list of option names in a C_CHOICES.

Now midend.c directly tests the returned pointer for equality to this
value, instead of checking whether it's the empty string.

A minor effect of this is that games may now return a dynamically
allocated empty string from interpret_move() and treat it as just
another legal move description. But I don't expect anyone to be
perverse enough to actually do that! The main purpose is that it
avoids returning a string literal from a function whose return type is
a pointer to _non-const_ char, i.e. we are now one step closer to
being able to make this code base clean under -Wwrite-strings.