regcomp: use unsigned char when testing for escapes

[yuripv]

Summary

• 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
Obtained from: NetBSD

[freebsd_igalic.co]

lib/libc/regex/regcomp.c

831

why doesn't GETNEXT() produce uch to begin with?

[yuripv]

lib/libc/regex/regcomp.c

831

You could probably try to change it and see what breaks :)

[yuripv]

Apparently NetBSD has a bit more complete fix for this (thanks Kyle), use it instead.

[kevans]

Thanks!