Replace my original asprintf() and vasprintf() hacks with something

Replace my original asprintf() and vasprintf() hacks with something
more cleanly integrated with stdio.  This should be faster and cleaner
since it doesn't memcpy() the data into a seperate buffer.  This lets
stdio allocate and manage the buffer and then hand it over to the user.

Obtained from: Todd Miller <Todd.Miller@courtesan.com> via OpenBSD