SSL information view

As an administrator, I find that you fairly often want to know what
your current connections are actually using as SSL parameters, and
there is currently no other way than gdb to find that out - something
we definitely should fix.

You can find it out today through libpq (the PQgetssl functions), the
psql banner, or contrib/sslinfo. All of them are client side (the
sslinfo module runs on the server, but it's just SQL functions that
can show information about the current connection, nothing that can be
used to inspect other connections).

I recently put together a quick hack
(https://github.com/mhagander/pg_sslstatus) that exposes a view with
this information, but it's definitely hacky, and it really is
functionality that we should include in core. Thus, I'll provide a
version of that hack for 9.5.

Before doing that, however, I'd like to ask for opinions :) The hack
currently exposes a separate view that you can join to
pg_stat_activity (or pg_stat_replication) on the pid -- this is sort
of the same way that pg_stat_replication works in the first place. Do
we want something similar to that for a builtin SSL view as well, or
do we want to include the fields directly in pg_stat_activity and
pg_stat_replication?

Second, I was planning to implement it by adding fields to
PgBackendStatus and thus to BackendStatusArray, booleans directly in
the struct and strings similar to how we track for example hostnames.
Anybody see a problem with that?

--
Magnus Hagander
Me: http://www.hagander.net/
Work: http://www.redpill-linpro.com/