TLS is currently implemented over either OpenSSL or GnuTLS, with more
backends likely to appear in the future. Currently, those backend libraries
are part of the protocol names used during e.g. the configure stage of a
build. Hide those details behind a generically-named declaration for the
TLS protocol to avoid leaking those details into the configuration stage.
Instead of a linked list constructed at av_register_all(), store them
in a constant array of pointers.
Since no registration is necessary now, this removes some global state
from lavf. This will also allow the urlprotocol layer caller to limit
the available protocols in a simple and flexible way in the following
commits.
