psql: Disable %P (pipeline status) for non-active connection

psql: Disable %P (pipeline status) for non-active connection

In the psql prompt, %P prompt shows the current pipeline status.  Unlike
most of the other options, its status was showing up in the output
generated even if psql was not connected to a database.  This was
confusing, because without a connection a pipeline status makes no
sense.

Like the other options, %P is updated so as its data is now hidden
without an active connection.

Author: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/86EF76B5-6E62-404D-B9EC-66F4714D7D5F@gmail.com
Backpatch-through: 18