nspawn,pid1: pass "inaccessible" nodes from cntr mgr to pid1 payload via /run/host

Let's make /run/host the sole place we pass stuff from host to container
in and place the "inaccessible" nodes in /run/host too.

In contrast to the previous two commits this is a minor compat break, but
not a relevant one I think. Previously the container manager would place
these nodes in /run/systemd/inaccessible/ and that's where PID 1 in the
container would try to add them too when missing. Container manager and
PID 1 in the container would thus manage the same dir together.

With this change the container manager now passes an immutable directory
to the container and leaves /run/systemd entirely untouched, and managed
exclusively by PID 1 inside the container, which is nice to have clear
separation on who manages what.

In order to make sure systemd then usses the /run/host/inaccesible/
nodes this commit changes PID 1 to look for that dir and if it exists
will symlink it to /run/systemd/inaccessible.

Now, this will work fine if new nspawn and new pid 1 in the container
work together. as then the symlink is created and the difference between
the two dirs won't matter.

For the case where an old nspawn invokes a new PID 1: in this case
things work as they always worked: the dir is managed together.

For the case where different container manager invokes a new PID 1: in
this case the nodes aren't typically passed in, and PID 1 in the
container will try to create them and will likely fail partially (though
gracefully) when trying to create char/block device nodes. THis is fine
though as there are fallbacks in place for that case.

For the case where a new nspawn invokes an old PID1: this is were the
(minor) incompatibily happens: in this case new nspawn will place the
nodes in the /run/host/inaccessible/ subdir, but the PID 1 in the
container won't look for them there. Since the nodes are also not
pre-created in /run/systed/inaccessible/ PID 1 will try to create them
there as if a different container manager sets them up. This is of
course not sexy, but is not a total loss, since as mentioned fallbacks
are in place anyway. Hence I think it's OK to accept this minor
incompatibility.

src/test/test-dev-setup.c

@@ -3,6 +3,7 @@
 #include "capability-util.h"
 #include "dev-setup.h"
 #include "fs-util.h"
+#include "mkdir.h"
 #include "path-util.h"
 #include "rm-rf.h"
 #include "tmpfile-util.h"
@@ -17,8 +18,8 @@ int main(int argc, char *argv[]) {
 
         assert_se(mkdtemp_malloc("/tmp/test-dev-setupXXXXXX", &p) >= 0);
 
-        f = prefix_roota(p, "/run");
-        assert_se(mkdir(f, 0755) >= 0);
+        f = prefix_roota(p, "/run/systemd");
+        assert_se(mkdir_p(f, 0755) >= 0);
 
         assert_se(make_inaccessible_nodes(f, 1, 1) >= 0);