regcomp: use unsigned char when testing for escapes

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