Use POSIX regcomp/regexec.

A temporary measure, since POSIX regexes (a) don't have the same
syntax as javascript's regexes, and (b) don't work on UTF-8 (unless
the setlocale crap is configured correctly, and the implementation
supports it), and (c) doesn't work on windows.
Add js_pushlstring to avoid a lot of try/malloc/pushstring boilerplate.
String.replace()