Unlocalized isspace and friends

Do our own ctype.h, just to get the sane semantics: we want
locale-independence, _and_ we want the right signed behaviour. Plus we
only use a very small subset of ctype.h anyway (isspace, isalpha,
isdigit and isalnum).

Signed-off-by: Junio C Hamano <junkio@cox.net>