utils/sexp: add some convenience

Add some convenience function for dealing the head/tail
Some whitespace / cosmetics
Add operators ==
Add operator !=
Update command handler for this.

lib/utils/mu-command-handler.cc

@@ -93,7 +93,7 @@ validate(const CommandHandler::CommandInfoMap& cmap,
 				 [&](auto&& arg) { return cmdargname == arg.first; }))
 			return Err(Error::Code::Command,
 				   "unknown parameter '%s 'in command '%s'",
-				   cmdargname.c_str(), cmd.to_string().c_str());
+				   cmdargname.name.c_str(), cmd.to_string().c_str());
 	}
 
 	return Ok();