Tab completion for SET TimeZone

Hi hackers,

I noticed there was no tab completion for time zones in psql, so here's
a patch that implements that.  I chose lower-casing the names since they
are case insensitive, and verbatim since ones without slashes can be
entered without quotes, and (at least my version of) readline completes
them correctly if the entered value starts with a single quote.

- ilmari