Add systemd support to relevant services

Add full systemd support for (at least) fedora 15-19, plus rawhide.
Other systemd-based distributions may work, but have not been tested.
Additionally, update RPM spec file to package RPMs for the
above-referenced fedora distributions.

Specific changes:

 - add new --with-systemd-unitdir AC_ARG, defaulting to
   /usr/lib/systemd/system
 - add systemd unit files in $systemd-unitdir for icinga and ido2db
 - add new --with-systemd-sysconfigdir AC_ARG, defaulting to
   /etc/sysconfig
 - add new $systemd-sysconfigdir/icinga environment file to provide
   configuration for the icinga and ido2db systemd services
 - update RPM spec file to build systemd-enabled packages on systemd-
   enabled fedora distributions, without breaking backward compatibility

fixes #3447