(searchbufs): New variable, replaces searchbuf and last_regexp and

(searchbufs): New variable, replaces searchbuf and last_regexp and
search_fastmap.
(syms_of_search): Initialize it.
(searchbuf_head): New variable.
(compile_pattern_1): Renamed from compile_pattern and made static.
Copy the regexp string in case the caller modifies it in place later.
(compile_pattern): New code, uses a cached entry if one is available.
The pattern buffer is now a return value instead of an arg.
(set_search_regs): Set num_regs directly, since there's no active
pattern buffer to pass to re_set_registers.
(Fstore_match_data): Likewise.
(Flooking_at): Use the new calling convention for compile_pattern.
(Fstring_match, fast_string_match, search_buffer): Likewise.