Fix preun and postun init script management in spec file

This fixes the issue when uninstalling package from Sys V init
based system. It called chconfig after the main init file was
already gone. That call then obviously failed.

It also adds the systemd preun management macro.

Change-Id: Ife5b948d0e3a58abea67e30ec5c300938bbb3c18
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1055156
Signed-off-by: Martin Sivak <msivak@redhat.com>