core: collapse JOB_RELOAD on an inactive unit into JOB_NOP

Before this commit systemctl reload on an inactive unit with a queued
start job would block until the unit had started if the unit supported
reload, but return failure immediately if the unit didn't.

Additionally systemctl reload-or-try-restart (and systemctl force-reload)
would block until the unit had started if the unit supported reload, but
return *success* immediately if the unit didn't.

Finaly reload on an inactive unit without a queued start job would
reurn failure, but try-restart on the same job would return success.

This behaviour is unintuitive and inconsistent, so make reload of inactive
units behave just like try-restart.

Also fixes deadlocks on boot if a unit calls systemctl reload,
reload-or-try-restart or force-reload on a unit ordered later
in the boot sequence during startup.
manager: avoid restart jobs when shutting down.

Doing so won't work and makes no sense.

See http://bugs.debian.org/624599 and http://bugs.debian.org/635777
manager: avoid restart jobs that can lead to deadlocks during early boot

See http://bugs.debian.org/624599
sysv-generator: do not generate 'Wants' symlinks to generated service files that will be shadowed by a native unit.