Renaming _C and friends to _CTYPE_C

[Joerg Sonnenberger <joerg%britannica.bec.de@localhost>]

Hi all,
our ctype.h uses the very generic single upper letter macro names for
the ctype masks. That is bad as it is also useful for naming function
arguments in header files etc. Any objection to consistently rename them
by prefixing them with _CTYPE?

Joerg