dropdb and dropuser: IF EXISTS

I noticed a few places where it would be handy if dropdb took a flag
like "--if-exists" which would basically just add in the 'IF EXISTS'
clause to the DROP DATABASE statement. For example, scripts like
find_static or mbregress.sh use dropdb && createdb, but they generate
noisy errors from dropdb when run for the first time since there's no
--if-exists flag. (They could just pipe 'DROP DATABASE IF EXISTS ...'
to psql, but what's the point of having dropdb if it's not used?)

Attached is a very quick patch implementing the "--if-exists" or "-X"
option for dropdb and dropuser. I didn't bother adding in a check to
make sure the server version was 8.2+ since we're not even supporting
8.1 nowadays, though that'd be easy enough to add in.

Josh