logind: always check for inhibitor locks

Currently inhibitors are bypassed unless an explicit request is made to
check for them, or even in that case when the requestor is root or the
same uid as the holder of the lock.

But in many cases this makes it impractical to rely on inhibitor locks.
For example, in Debian there are several convoluted and archaic
workarounds that divert systemctl/reboot to some hacky custom scripts
to try and enforce blocking accidental reboots, when it's not expected
that the requestor will remember to specify the command line option
to enable checking for active inhibitor locks.

Also in many cases one wants to ensure that locks taken by a user are
respected by actions initiated by that same user.

Change logind so that inhibitors checks are not skipped in these
cases, and systemctl so that locks are checked in order to show a
friendly error message rather than "permission denied".

Add new block-weak and delay-weak modes that keep the previous
behaviour unchanged.

docs/INHIBITOR_LOCKS.md

@@ -42,6 +42,9 @@ If such a lock is taken the operation will fail (but still may be overridden if
 The InhibitDelayMaxSec= setting in [logind.conf(5)](http://www.freedesktop.org/software/systemd/man/logind.conf.html) controls the timeout for this. This is intended to be used by applications which need a synchronous way to execute actions before system suspend but shall not be allowed to block suspend indefinitely.
 This mode is only available for _sleep_ and _shutdown_ locks.
 
+3. _block-weak_ and _delay-weak_ that work as the non-weak counterparts, but that in addition may be ignored
+automatically and silently under certain circumstances, unlike the formers which are always respected.
+
 Inhibitor locks are taken via the Inhibit() D-Bus call on the logind Manager object:
 
 ```