Add libpq function PQhostaddr().

There was a bug in the psql's meta command \conninfo. When the
IP address was specified in the hostaddr and psql used it to create
a connection (i.e., psql -d "hostaddr=xxx"), \conninfo could not
display that address. This is because \conninfo got the connection
information only from PQhost() which could not return hostaddr.

This patch adds PQhostaddr(), and changes \conninfo so that it
can display not only the host name that PQhost() returns but also
the IP address which PQhostaddr() returns.

The bug has existed since 9.1 where \conninfo was introduced.
But it's too late to add new libpq function into the released versions,
so no backpatch.