core: rename/generalize UNIT(u)->test_start_limit() hook

core: rename/generalize UNIT(u)->test_start_limit() hook

Up until now the main reason why we didn't proceed with starting the
unit was exceed start limit burst. However, for unit types like mounts
the other reason could be effective ratelimit on /proc/self/mountinfo
event source. That means our mount unit state may not reflect current
kernel state. Hence, we need to attempt to re-run the start job again
after ratelimit on event source expires.

As we will be introducing another reason than start limit let's rename
the virtual function that implements the check.