Use config_parse_sec_fix_0() also for JobRunningTimeoutSec

2d79a0bb did that for TimeoutSec=,
89beff89 did that for JobTimeoutSec=,
and 0004f698 did that for
x-systemd.device-timeout=. But after parsing x-systemd.device-timeout=xxx
we write it out as JobRunningTimeoutSec=xxx. Two options:
- write out JobRunningTimeoutSec=<a very big number>,
- change JobRunningTimeoutSec= to behave like the other options.

I think it would be confusing for JobRunningTimeoutSec= to have different
syntax then TimeoutSec= and JobTimeoutSec=, so this patch implements the
second option.

Fixes #6264, https://bugzilla.redhat.com/show_bug.cgi?id=1462378.

(cherry picked from commit 4a06cbf8)