Support "ssl.chain" in Aphlict configuration

Summary: Fixes T10806. Although browsers don't seem to care about this, it's more correct to support it, and the new test console uses normal cURL and does care.

Test Plan:

• Hit the error case for providing a chain but no key/cert.
• Used openssl s_client -connect localhost:22280 to connect to local Aphlict servers.
• With SSL but no chain, saw openssl fail to verify the remote.
• With SSL and a chain, saw openssl verify the identify of the remote.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10806

Differential Revision: https://secure.phabricator.com/D15709