pnp4nagios.spec: fix config fallback, only on fresh install; fix rpmlint warnings

+* Fri Feb 15 2013 Michael Friedrich <michael.friedrich@netways.de> -
0.6.19-4
+- if nagios is installed, use default config, otherwise fallback to
icinga
+- only sed config in sysconfdir on fresh installs, upgrades are
not-to-be overridden
+- fix rpmlint warnings