Add support for setting the SSL CA location

This is a re-worked version of pull request #2583. As discussed there, the only logical way to set the OpenSSL CA cert location is through `git_libgit2_opts`, since this is a global setting that affets the whole library. While it would be nice to have a separate OpenSSL context per connection that pulls its settings from the config variables `sslcainfo` and `sslcapath`, doing this doesn't seem to be possible.