core: add "tls-ca-path" option

Some organizations will want to use private Certificate Authorities to
serve content to their clients.  While it's possible to add the CA
to the system-wide CA store, that has two drawbacks:

1) Compromise of that cert means it can be used for other web traffic
2) All of ca-certificates is trusted

This patch allows a much stronger scenario where *only* the CAs in
tls-ca-path are used for verification from the given repository.

https://bugzilla.gnome.org/show_bug.cgi?id=726256