everywhere: remove configurability of sysv runlevel to target mapping

With this change runlevel 2, 3, 4 are mapped to multi-user.target for
good, and 5 to graphical.target. This was already the previous mapping
but is now no longer reconfigurable, but hard-coded into the core.

This should generally simplify things, but also fix one bug: the
sysv-generator previously generated symlinks to runlevel[2-5].target
units, which possibly weren't picked up if these aliases were otherwise
only referenced by the real names "multi-user.target" and
"graphical.target".

We keep compat aliases "runlevel[2345].target" arround for cases where
this target name is explicitly requested.