[patch] Add support for connection strings to createuser and dropuser

All of the other CLI tools seems to support connection strings via the
PGDATABASE environment variable and the --maintenance-db command line
option, but this functionality is missing from createuser and dropuser.
Attached is a patch to bring them up to par with the other CLI tools.

There is an argument to be made for renaming or aliasing the
--maintenance-db parameter as --connection-string or similar for all of
these tools of course, but all of the others (except createdb and dropdb
afaics) accepts a "raw" connection string on the command line so it
might not matter much for them.

--
Anders Åstrand
Percona