Add systemd buildrequires and requires
First try at a systemd service definition file, install it from CMake.

Native systemd support is enabled with cmake -DUSE_SYSTEMD=ON

Refs #4794
Fix CMake escaping.

Refs #4794
Systemd can't resolve variables recursively.

Refs #4794
Reloading with systemd is fixed now.

Refs #4794
Unify resources for SysV-Init and systemd.

- Move system-specific defines like paths and usernames to /etc/icinga2/sysdefines.conf
  Do not use /etc/sysconfig for this as per suggestion on the systemd mailinglist: it is RedHat-specific
- Use /etc/icinga2/sysdefines.conf in SysV-Init and systemd
- Move both the sources of the SysV-Initscript and the systemd-service definition to etc/initsystem

Refs #4794

Conflicts:
	etc/initsystem/icinga2.init.d.cmake
Move code preparing dirs and permissions to icinga2-prepare-dirs, use this for SysV-Init and systemd.

Refs #4794

Conflicts:
	etc/initsystem/icinga2.init.d.cmake
Generate the service unit on Fedora and use the %systemd_post/_postun/_preun macros for service setup

Refs #4794
Remove mention of icinga in the description since it's already in the name and might cause versioning confusion

Refs #4794
Start with sysvinit installation logic
Add missing endif
Move decision about systemd/sysvinit to one location. Add files forgotten during merge.

Refs #4794
Move the sysdefines.conf file to /etc/sysconfig.

As /etc/sysconfig is distro specific, the filename used can be configured via the CMake parameter
ICINGA2_SYSCONFIGFILE

Refs #4794
Add a manpage for icinga2-prepare-dirs.

Refs #4794
Don't mark the systemd service file as executable.

Refs #4794
Don't call check_run in the initscript, this function is gone.

Refs #4794
install new man page properly

refs #4794
Merge branch 'feature/systemd-4794-final' into next

fixes #4794
update AUTHORS