Move last few URL functions into http.c

This makes it easier to split http.c and friends into their own
 library.
This allows distributions to ship a libopusfile with generic Opus
 parsing support, and a libopusurl with http/https support.
Keeping the latter in a separate library means that GPL
 applications don't have to link against the GPL-incompatible
 openssl, and distributions don't have to disable http support to
 allow GPL applications to use libopusfile.