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.
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