add option -initonly to "mox localserve", to only create config files and then quit

for issue #89 by naturalethic

doc.go

@@ -346,6 +346,8 @@ during those commands instead of during "data".
 	usage: mox localserve
 	  -dir string
 	    	configuration storage directory (default "$userconfigdir/mox-localserve")
+	  -initonly
+	    	write configuration files and exit
 	  -ip string
 	    	serve on this ip instead of default 127.0.0.1 and ::1. only used when writing configuration, at first launch.