sysv-generator: initialize units before use to ensure correct ordering

The original loop called fix_order() on each service immediately after
loading it, but fix_order() would reference other units which were not
loaded yet.
This resulted in bogus and unnecessary orderings based on the static
start priorities.

Therefore call load_sysv() for every init script when traversing them in
enumerate_sysv(). This ensures that all units are loaded when
fix_order() is called.

Bug-Debian: https://bugs.debian.org/771118