fix bug 1079957 - RPM after-install should su postgres, not use network ...

...connection
Although people do this all the time, I prefer passing `-q` to `grep` rather than redirecting to `/dev/null`, as it makes the intention more explicit (imho).

As an aside, it's perhaps worth noting that this will execute in `pwd`, so if user `postgres` doesn't have `+rw` on said, a warning will be generated on STDOUT.  I don't imagine this is a problem during the package install process, but it did cause me to waste some time debugging it, heh. :mag: