Fix psql \d commands to behave properly when a pattern using regex | is given.

Fix psql \d commands to behave properly when a pattern using regex | is given.
Formerly they'd emit '^foo|bar$' which is wrong because the anchors are
parsed as part of the alternatives; must emit '^(foo|bar)$' to get expected
behavior.  Same as bug found previously in similar_escape().  Already fixed
in HEAD, this is just back-porting the part of that patch that was a bug fix.