Validate hex strings w/o PCRE. Code cleanup.

- normalize_hexstring() was checking for valid hex strings w/o PCRE
  *already*, just let it return NULL if the hexstring wasn't valid.
- remove the (PCRE-based) validate_hexstring() altogether
- move all exit() calls to remembyte.c. everything else should return
  a failure, not exit the program.
- more consistent error checking