* charset.h (DECODE_CHAR): Return int, not unsigned;

* charset.h (DECODE_CHAR): Return int, not unsigned;

this is what was intended anyway, and it avoids undefined behavior.
(CHARSET_OFFSET): Remove unused macro, instead of fixing its
integer-overflow issues.
(ENCODE_CHAR): Return unsigned on all hosts, not just on 32-bit hosts.
Formerly, it returned EMACS_INT on 64-bit hosts in the common case
where the argument is EMACS_INT, and this behavior is not intended.