Fix regex type usage

Fix regex type usage

include/regex.h had not been updated during the int -> Idx transition,
and the prototypes don't matched the definitions in regexec.c.

In regcomp.c, most interfaces were updated for Idx, except for two ones
guarded by #if _LIBC.

Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>