cli: help command now also prints usage

The help command now adds command usage to its output by calling each
command handler in CMD_USAGE mode.

Instead of seeing, for example:

	decodepay
	    Decode {bolt11}, using {description} if necessary

we see:

	decodepay bolt11 [description]
	    Decode {bolt11}, using {description} if necessary

Signed-off-by: Mark Beckwith <wythe@intrig.com>

tests/test_misc.py

@@ -622,7 +622,7 @@ def test_cli(node_factory):
                                    .format(l1.daemon.lightning_dir),
                                    'help']).decode('utf-8')
     # Test some known output.
-    assert 'help\n    List available commands, or give verbose help on one command' in out
+    assert 'help [command]\n    List available commands, or give verbose help on one {command}' in out
 
     # Test JSON output.
     out = subprocess.check_output(['cli/lightning-cli',