Minor fixes for signed vs unsigned integers.

Minor fixes for signed vs unsigned integers.

* character.h (MAYBE_UNIFY_CHAR):
* charset.c (maybe_unify_char):
* keyboard.c (read_char, reorder_modifiers):
XINT -> XFASTINT, since the integer must be nonnegative.
* ftfont.c (ftfont_spec_pattern):
* keymap.c (access_keymap, silly_event_symbol_error):
XUINT -> XFASTINT, since the integer must be nonnegative.
(Fsingle_key_description, preferred_sequence_p): XUINT -> XINT,
since it makes no difference and we prefer signed.
* keyboard.c (record_char): Use XUINT when all the neighbors do.
(access_keymap): NATNUMP -> INTEGERP, since the integer must be
nonnegative.