machine: introduce io.systemd.Machine.BindMount method (#35066)

This PR introduces io.systemd.Machine.BindMount method which is
alternative to DBus's BindMountMachine.

test/units/TEST-13-NSPAWN.machined.sh

@@ -401,6 +401,8 @@ varlinkctl call /run/systemd/machine/io.systemd.Machine io.systemd.Machine.Open
 timeout 30 bash -c "until test -e /tmp/none-existent-file; do sleep .5; done"
 grep -q "BAR" /tmp/none-existent-file
 
+# io.systemd.Machine.BindMount is covered by testcase_check_machinectl_bind() in nspawn tests
+
 # terminate machines
 machinectl terminate long-running
 # wait for the container being stopped, otherwise acquiring image metadata by io.systemd.MachineImage.List may fail in the below.

test/units/TEST-13-NSPAWN.nspawn.sh

@@ -796,7 +796,7 @@ EOF
 
 testcase_machinectl_bind() {
     local service_path service_name root container_name ec
-    local cmd='for i in $(seq 1 20); do if test -f /tmp/marker; then exit 0; fi; sleep .5; done; exit 1;'
+    local cmd='for i in $(seq 1 20); do if test -f /tmp/marker && test -f /tmp/marker-varlink; then exit 0; fi; sleep .5; done; exit 1;'
 
     root="$(mktemp -d /var/lib/machines/TEST-13-NSPAWN.machinectl-bind.XXX)"
     create_dummy_container "$root"
@@ -814,6 +814,8 @@ EOF
     systemctl start "$service_name"
     touch /tmp/marker
     machinectl bind --mkdir "$container_name" /tmp/marker
+    touch /tmp/marker-varlink
+    varlinkctl call /run/systemd/machine/io.systemd.Machine io.systemd.Machine.BindMount "{\"name\": \"$container_name\", \"source\": \"/tmp/marker-varlink\", \"mkdir\": true}"
 
     timeout 10 bash -c "while [[ '\$(systemctl show -P SubState $service_name)' == running ]]; do sleep .2; done"
     ec="$(systemctl show -P ExecMainStatus "$service_name")"