* regex.c: Declare a new type `re_char' used throughout the code for the

* regex.c: Declare a new type `re_char' used throughout the code for the
string char type.  It's `const unsigned char' to match the rest of Emacs.
Consistently make sure all pointers to strings use it and make sure all
pointers into the pattern use `unsigned char'.
(re_match_2_internal): Use `PREFETCH+STRING_CHAR' instead of
GET_CHAR_AFTER_2.
Also merge wordbound and notwordbound to reduce code duplication.
* charset.h (GET_CHAR_AFTER_2): Remove.
(GET_CHAR_BEFORE_2): Use unsigned chars, like everywhere else.