ejabberdctl: Omit su(1)'s "-p" flag

On Linux, su(1)'s "-p" flag makes sure the following environment
variables are preserved: $HOME, $SHELL, $USER, and $LOGNAME.  The flag
isn't portable, and since we don't set HOME=$SPOOLDIR anymore, there's
no reason to preserve these variables anyway.

Without "-p", we also don't need to set HOME=$INSTALLUSER_HOME, as su(1)
now does that for us.
Merge pull request #264 from weiss/su-without-p

ejabberdctl: Omit su(1)'s "-p" flag