vmspawn: Add --user/--system and support user session machined registration

The UX of registering with the user session machined
instance is much better as there won't be an authorization
prompt. To make that available for users, let's add --user
and --system switches for vmspawn. For backwards compat, we'll
still try to register with the system machined instance if the
user machined instance is not available.

shell-completion/bash/systemd-vmspawn

@@ -29,7 +29,7 @@ _systemd_vmspawn() {
     local i verb comps
 
     local -A OPTS=(
-        [STANDALONE]='-h --help --version -q --quiet --no-pager -n --network-tap --network-user-mode'
+        [STANDALONE]='-h --help --version -q --quiet --no-pager -n --network-tap --network-user-mode --user --system'
         [PATH]='-D --directory -i --image --linux --initrd --extra-drive --forward-journal'
         [BOOL]='--kvm --vsock --tpm --secure-boot --discard-disk --register --pass-ssh-key'
         [FIRMWARE]='--firmware'