Fix buffer management in strbuf_catfv.

Fix buffer management in strbuf_catfv.

Thanks to Tim Kosse for pointing out that I had _completely_ cocked up
all the code that was supposed to enlarge the buffer in the strbuf
structure, by failing to pass in 'oldsize' to the innermost
dupvprintf_inner function by reference, so that the size was never
updated.

Fortunately, this whole mechanism was something I dashed off for the
purposes of buildinfo(), which means it's only ever used to glue
together a fixed number of compile-time string constants, for which
there turns out to be plenty to spare in the standard 512 bytes
allocated to a new strbuf. So it's at least not dangerous, though it
clearly needs to be fixed before I make the mistake of using
strbuf_catf[v] for anything else!