units/systemd-sysctl.service.in: run after load-modules

Modules might or will register new sysctl options.

[zj: This mechanism of adding modules just to reliably set sysctl
attributes is not ideal. Nevertheless, sysctl for dynamically created
attributes is simply broken, and this is the easiest workaround.]

https://bugzilla.redhat.com/show_bug.cgi?id=1022977
https://bugzilla.novell.com/show_bug.cgi?id=725412
man: document statically loading modules for sysctl settings

https://bugzilla.redhat.com/show_bug.cgi?id=1022977
https://bugzilla.novell.com/show_bug.cgi?id=725412