unit: when JobTimeoutSec= is turned off, implicitly turn off JobRunningTimeoutSec= too

We added JobRunningTimeoutSec= late, and Dracut configured only
JobTimeoutSec= to turn of root device timeouts before. With this change
we'll propagate a reset of JobTimeoutSec= into JobRunningTimeoutSec=,
but only if the latter wasn't set explicitly.

This should restore compatibility with older systemd versions.

Fixes: #6402