improve alloc_vprintf

improve alloc_vprintf

The previous implementation was unnecessarily complex. Get rid of the loops,
let vsnprintf() tell us directly how much storage we need and allocate that. A
second pass writes the actual string. Also add a va_end() that was missing.
This should be much faster for large strings and less wasteful for small ones.

A quirk that has been retained is that some callers patch in a newline at the
end of the returned string and depend on alloc_vprintf to allocate at least
one byte extra.

Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Signed-off-by: Zachary T Welch <zw@superlucidity.net>