core: replace OnFailureIsolate= setting by a more generic OnFailureJobMode= setting and make use of it where applicable
core: add new "flush" job mode to cancel all other jobs when queuing a new job
systemctl: replace the three job mode options by a single --job-mode= option

Also, expose the new "flush" job mode this way.
man: explain the precise syntax and feature set of unit names at the end of the man page, given that it is detail information