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.