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

...connection
It is perhaps worth adding a little extra error checking here just in case the previous command exits non-zero for reasons other than the empty grep result list.  For example, we could check to make sure that `psql -l` produces some sane output first, then check for the `breakpad` string as a second test, or just capture the output of the previous and grep for the latter.