Add a 'strbuf' system, for building up a large string piece by piece.

Add a 'strbuf' system, for building up a large string piece by piece.

I'm faintly surprised I haven't needed this before. Basically it's an
allocating string formatter, like dupprintf, except that it
concatenates on to the end of a previous string. You instantiate a
strbuf, then repeatedly call strbuf_catf to append pieces of formatted
output to it, and then you can extract the whole string and free it
(separately or both in one step).