connect: add function to fetch value of a v2 server capability

connect: add function to fetch value of a v2 server capability

So far in protocol v2, all of our server capabilities that have values
have not had values that we've been interested in parsing.  For example,
we receive but ignore the agent value.

However, in a future commit, we're going to want to parse out the value
of a server capability.  To make this easy, add a function,
server_feature_v2, that can fetch the value provided as part of the
server capability.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>