systemd: Corrected an issue introduced in the last systemd.plugin commit

Installwatch log is no longer around during BUILD_POST_INSTALL stage which
meant unit files was not properly tracked with the introduced change. The
fix was to split out the daemon restart part into its own function which
then is properly called in BUILD_POST_INSTALL. Installation and
enabling of services is once again running in BUILD_POST_BUILD stage.
Merge branch 'systemd_plugin_fix'

Solves the problem at hand