(struct regexp_cache): New field `posix'.

(struct regexp_cache): New field `posix'.
(compile_pattern_1): New arg `posix'.  Controls the syntax spec
for parsing; also record it in the cache.
(compile_pattern): New arg `posix'.
(looking_at_1, string_match_1, fast_string_match): Pass 0 for new arg.
(search_buffer): New arg `posix'.  Now static.
(search_command): New arg `posix'.  All callers changed.
(Fposix_search_forward, Fposix_search_backward): New functions.
(string_match_1, looking_at_1): New subroutines broken out.
(Fstring_match, Flooking_at): Use them.
(Fposix_string_match, Fposix_looking_at): New functions.
(syms_of_search): defsubr new functions.