core: support MountAPIVFS and RootDirectory in user manager

The only piece missing was to somehow make /proc appear in the
new user+mount namespace. It is not possible to mount a new
/proc instance, not even with hidepid=invisible,subset=pid, in
a user namespace unless a PID namespace is created too (and also
at the same time as the other namespaces, it is not possible to
mount a new /proc in a child process that creates a PID namespace
forked from a parent that created a user+mount namespace, it has
to happen at the same time).

Use the host's /proc with a bind-mount as a fallback for this
case. User session services would already run with it, so
nothing is lost.

test/units/testsuite-43.sh

@@ -68,6 +68,24 @@ runas testuser systemd-run --wait --user --unit=test-group-fail \
     -P true \
     && { echo 'unexpected success'; exit 1; }
 
+# Check that with a new user namespace we can bind mount
+# files and use a different root directory
+runas testuser systemd-run --wait --user --unit=test-bind-mount \
+    -p PrivateUsers=yes -p BindPaths=/dev/null:/etc/os-release \
+    test ! -s /etc/os-release
+
+unsquashfs -no-xattrs -d /tmp/img /usr/share/minimal_0.raw
+runas testuser systemd-run --wait --user --unit=test-root-dir \
+    -p PrivateUsers=yes -p RootDirectory=/tmp/img \
+    grep MARKER=1 /etc/os-release
+
+mkdir /tmp/img_bind
+mount --bind /tmp/img /tmp/img_bind
+runas testuser systemd-run --wait --user --unit=test-root-dir-bind \
+    -p PrivateUsers=yes -p RootDirectory=/tmp/img_bind \
+    grep MARKER=1 /etc/os-release
+umount /tmp/img_bind
+
 systemd-analyze log-level info
 
 echo OK >/testok