add systemd unit file

Update the spec file with the rpm macro to put the unit file
in the system-wide unit file directory based on:

    [root@1713bbf19a0b /]# rpmbuild --eval '%{_unitdir}'
    /usr/lib/systemd/system

Allow user to create a drop directory to specify options
via environment variables.

Resolves https://github.com/purpleidea/mgmt/issues/12.

spec.in

@@ -40,6 +40,8 @@ rm -rf %{buildroot}
 # _datadir is typically /usr/share/
 install -d -m 0755 %{buildroot}/%{_datadir}/__PROGRAM__/
 cp -a AUTHORS COPYING COPYRIGHT DOCUMENTATION.md README.md THANKS examples/ %{buildroot}/%{_datadir}/__PROGRAM__/
+mkdir -p %{buildroot}/%{_unitdir}/
+install -pm 0644 misc/__PROGRAM__.service %{buildroot}/%{_unitdir}/
 
 # install the binary
 mkdir -p %{buildroot}/%{_bindir}
@@ -58,6 +60,7 @@ install -m 0644 misc/example.conf %{buildroot}%{_sysconfdir}/__PROGRAM__/__PROGR
 %{_datadir}/__PROGRAM__/*
 %{_bindir}/__PROGRAM__
 %{_sysconfdir}/__PROGRAM__/*
+%{_unitdir}/__PROGRAM__.service
 
 # this changelog is auto-generated by git log
 %changelog