service-management: if previous code was compiled with ENABLE_SYSTEMD,

the systemd service manager will be the current service manager,
even that systemd wasn't active.

Signed-off-by: Juhász Viktor <jviktor@balabit.hu>
Merge pull request #327 from juhaszviktor/f/service-manager-fix

service-management: if previous code was compiled with ENABLE_SYSTEMD,