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