[MASSMAIL]psql: Greatly speed up "\d tablename" when not using regexes

Running "\d tablename" from psql could take multiple seconds when
running on a system with 100k+ tables. The reason for this was that
a sequence scan on pg_class takes place, due to regex matching being
used.

Regex matching is obviously unnecessary when we're looking for an exact
match. This checks for this (common) case and starts using plain
equality in that case.