build-sys: move remaining tools from sbin/ to bin/ since they might eventually be useful for user execution

Now that we have /dev/.run there's no need to use abstract namespace
sockets. So, let's move things to /dev/.run, to make things more easily
discoverable and improve compat with chroot() and fs namespacing.

ERROR: files left in build directory after distclean:
./src/org.freedesktop.systemd1.policy
make[1]: *** [distcleancheck] Error 1

This adds support for executing systemctl operations remotely or as
privileged user while still running systemctl itself unprivileged and
locally.

This currently requires a D-Bus patch to work properly.

https://bugs.freedesktop.org/show_bug.cgi?id=35230

<juergbi> VALA_0_X is defined up to and including the current version
<juergbi> so VALA_0_10 is defined in 0.10 and 0.11 and 0.12
<juergbi> VALA_0_12 is defined in 0.11 and 0.12 (and later versions)
<juergbi> reverse the branches and use VALA_0_12 as conditional
<kay> juergbi: changing api but keep the defines?
<kay> how do we support vala 0.14 then?
<kay> you keep that model with the old defines?
<juergbi> VALA_0_10 corresponds to valaversion >= 0.10

If flock fails, fd is not returned to caller so it cannot clean up.

main: don't check if /usr really is a mount point, since it is fine if it is passed pre-mounted to us from the initrd