Add PQfullProtocolVersion() to surface the precise protocol version.

Add PQfullProtocolVersion() to surface the precise protocol version.

The existing function PQprotocolVersion() does not include the minor
version of the protocol.  In preparation for pending work that will
bump that number for the first time, add a new function to provide it
to clients that may care, using the (major * 10000 + minor)
convention already used by PQserverVersion().

Jacob Champion based on earlier work by Jelte Fennema-Nio

Discussion: http://postgr.es/m/CAOYmi+mM8+6Swt1k7XsLcichJv8xdhPnuNv7-02zJWsezuDL+g@mail.gmail.com