Use config_parse_sec_fix_0() also for JobRunningTimeoutSec

Use config_parse_sec_fix_0() also for JobRunningTimeoutSec

2d79a0bbb9f651656384a0a86ed814e6306fb5dd did that for TimeoutSec=,
89beff89edba592366b2960bd830d3f6e602c2c7 did that for JobTimeoutSec=,
and 0004f698df1410ef8b6ab3fb5f4b41a60c91182c 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.