systemctl: split up humungous systemctl.c file

This is just some refactoring: shifting around of code, not change in
codeflow.

This splits up the way too huge systemctl.c in multiple more easily
digestable files. It roughly follows the rule that each family of verbs
gets its own .c/.h file pair, and so do all the compat executable names
we support. Plus three extra files for sysv compat (which existed before
already, but I renamed slightly, to get the systemctl- prefix lik
everything else), a -util file with generic stuff everything uses, and a
-logind file with everything that talks directly to logind instead of
PID1.

systemctl is still a bit too complex for my taste, but I think this way
itc omes in a more digestable bits at least.

No change of behaviour, just reshuffling of some code.