core: convert to VERB_COMMAND and add --introspect-cli

--introspect-cli follows the same deferred pattern as --help: parse_argv()
only records the action, and it is executed in the same place where
ACTION_HELP and ACTION_VERSION are handled.

The help is looked up by explicit name: in the multicall configuration
the systemd and systemd-executor commands share one binary, so the
"first COMMAND" default would depend on link order.

The "This program takes no positional arguments." line is dropped from
the help output — the default "[OPTION…]" synopsis already says so.
(And also, this is not quite true. It does take positional args when
called as init.)

src/basic/proc-cmdline.c

@@ -37,6 +37,7 @@ int proc_cmdline_filter_pid1_args(char **argv, char ***ret) {
                 { "no-pager",                 no_argument,      },
                 { "help",                     no_argument,      },
                 { "version",                  no_argument,      },
+                { "introspect-cli",           no_argument,      },
                 { "dump-configuration-items", no_argument,      },
                 { "dump-bus-properties",      no_argument,      },
                 { "bus-introspect",           required_argument },