mount-tool: add support for a --json= output mode

Let's add a JSON output mode, like we have it for so many of our tools.

test/units/TEST-74-AUX-UTILS.mount.sh

@@ -23,6 +23,7 @@ systemd-mount --list --full
 systemd-mount --list --no-legend
 systemd-mount --list --no-pager
 systemd-mount --list --quiet
+systemd-mount --list --json=pretty
 
 # tmpfs
 mkdir -p "$WORK_DIR/mnt/foo/bar"