Implement message trailer parsing API

Upon downloading the pack file, the local transport will iterate through
every reference using `git_reference_foreach`. The function is a bit
tricky though in that it requires the passed callback to free the
references, which does not currently happen.

Fix the memory leak by freeing all passed references in the callback.

We might want OpenSSL to be the implementation for SHA-1 and/or TLS. If we only
want it for TLS (e.g. we're building with the collision-detecting SHA-1
implementation) then we did not indicate this to the systems including us a
static library.

Add OpenSSL to the list also during the TLS decision to make sure we say we
should link to it if we use it for TLS.

It is indeed a list of dependencies for those which include the static archive.
This is in preparation for adding two possible places where we might add openssl
as a dependency.

cmake: allow explicitly choosing SHA1 backend

Right now, if SHA1DC is disabled, the SHA1 backend is mostly chosen
based on which system libgit2 is being compiled on and which libraries
have been found. To give developers and distributions more choice,
enable them to request specific backends by passing in a
`-DSHA1_BACKEND=<BACKEND>` option instead. This completely replaces the
previous auto-selection.