Audit and replace all ctype.h uses

Something like "char *s = ...; isdigit(s[0]);" triggers undefined
behavior, because char can be signed, and thus s[0] can be a negative
value. The is*() functions require unsigned char _or_ EOF. EOF is a
special value outside of unsigned char range, thus the argument to the
is*() functions can't be a char.

This undefined behavior can actually trigger crashes if the
implementation of these functions e.g. uses lookup tables, which are
then indexed with out-of-range values.

Replace all <ctype.h> uses with our own custom mp_is*() functions added
with misc/ctype.h. As a bonus, these functions are locale-independent.
(Although currently, we _require_ C locale for other reasons.)

demux/demux_subreader.c

@@ -26,7 +26,6 @@
 #include <string.h>
 #include <sys/types.h>
 #include <dirent.h>
-#include <ctype.h>
 
 #include <libavutil/common.h>
 #include <libavutil/avstring.h>