Use standard isspace(3), isalpha(3), and isupper(3)

Use standard isspace(3), isalpha(3), and isupper(3)

Due to the recent removal of IN_CTYPE_DOMAIN(), the uppercase
macros that wrapped these standard calls are now defined to be
equivalent.  Therefore, there's no need for the wrappers, and it
is much more readable to use the standard calls directly.

However, hold on with ISDIGIT*(), since it's not so obvious what
to do with it.

Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>