regcomp: use unsigned char when testing for escapes

• cast GETNEXT to unsigned where it is being promoted to int to prevent sign-extension (really it would have been better for PEEK*() and GETNEXT() to return unsigned char; this would have removed a ton of (uch) casts, but it is too intrusive for now).
• fix an isalpha that should have been iswalpha

PR: 264275, 274032
Reviewed by: kevans, eugen (previous version)
Obtained from: NetBSD
Differential Revision: https://reviews.freebsd.org/D41947