regex-quote: New API. * lib/regex-quote.h: Include <stdbool.h>. (struct regex_quote_spec): New type. (regex_quote_spec_posix, regex_quote_spec_gnu, regex_quote_spec_pcre): New declarations. (regex_quote_length, regex_quote_copy, regex_quote): Take a 'const struct regex_quote_spec *' argument. * lib/regex-quote.c (RE_*, PCRE_*): New macros. (pcre_special): New constant. (regex_quote_spec_posix, regex_quote_spec_gnu, regex_quote_spec_pcre): New functions. (regex_quote_length, regex_quote_copy, regex_quote): Take a 'const struct regex_quote_spec *' argument. * modules/regex-quote (Depends-on): Add stdbool. * tests/test-regex-quote.c (check): Update for new API. Add test for anchored results. * NEWS: Mention the API change. Reported by Reuben Thomas and Eric Blake.