core: collapse JOB_RELOAD on an inactive unit into JOB_NOP [v2]

The old behavior is inconsistent and confusing, this commit simplifies matters significantly:

Before this commit:
  * systemctl try-restart on any inactive unit returns *success* immediately
  * systemctl reload on an inactive unit without a queued start job returns *failure* immediately
  * systemctl reload on an inactive unit supporting reload with a queued start job blocks until the unit is started
  * systemctl reload on an inactive unit not supporting reload with a queued start returns *failure* immediately
  * systemctl reload-or-try-restart on an inactive unit supporting reload blocks until the unit is started
  * systemctl reload-or-try-restart on an inactive unit not supporting reload returns *success* immediately

With this commit:
  * systemctl try-restart, reload and reload-or-try-restart on any inactive unit returns *success* immediately

By not blocking on a queued start job this also fixes a deadlock when
one job calls systemctl reload or reload-or-try-restart on a unit with
a start job ordered later in the transaction.
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
rules: skip 99-systemd.rules when not running systemd as init

Otherwise systemd-sysctl will be run even if systemd is not PID 1.
units: make it possible to disable tmp.mount using systemctl

But enable it by default in make install and systemd preset.
core: do not add default dependencies to /usr mount unit

This makes no difference if /usr was mounted in the initrd,
and brings the behaviour of legacy systems closer to those
with a propper initrd.
units: add RequiresMountsFor=/usr/… to various systemd service files

Unless both /usr and /usr/local is mounted in the initrd these
services might miss some of their configuration otherwise.