ctype: restore character classification for non-ASCII platforms

With commit 4272a0b0 curl-speficic
character classification macros and functions were introduced in
curl_ctype.[ch] to avoid dependencies on the locale. This broke curl on
non-ASCII, e.g. EBCDIC platforms. This change restores the previous set
of character classification macros when CURL_DOES_CONVERSIONS is
defined.

Closes #2494