Port recent character.h changes to --with-wide-int

Port recent character.h changes to --with-wide-int

* src/fns.c (mapcar1):
* src/keymap.c (Fkey_description):
* src/syntax.c (scan_lists):
Prefer ptrdiff_t to EMACS_INT where either will do; this fixes
newly-introduced type errors on --with-wide-int platforms where
ptrdiff_t is narrower than EMACS_INT.
* src/keymap.c (Fkey_description): Rework for clarity; remove goto.
* src/syntax.c (scan_words, Fforward_comment, scan_lists)):
Fix unlikely integer overflow problems that can occur on
--with-wide-int platforms, and that were caught by the recent
character.h changes.