command: use NUL for magic usage string

This change also improves documentation.

Signed-off-by: Ophestra <cat@gensokyo.uk>

command/builder.go

@@ -13,6 +13,7 @@ func New(output io.Writer, logf LogFunc, name string, early HandlerFunc) Command
 	return c
 }
 
+// newNode initialises a subcommand tree and returns its address.
 func newNode(output io.Writer, logf LogFunc, name, usage string) *node {
 	n := &node{
 		name: name, usage: usage,