Add display of oprcode (the underlying function's name) to psql's \do+.

The + modifier of \do didn't use to do anything, but now it adds an oprcode
column.  This is useful both as an additional form of documentation of what
the operator does, and to save a step when finding out properties of the
underlying function.

Marko Tiikkaja, reviewed by Rushabh Lathia, adjusted a bit by me