kill: print usage if no pid is passed in command line

This makes a command such as `kill -TERM` or `kill -9` fails and prints
usage, instead of silently succeeding.

The behavior is consistent with how `kill` behaves without an explicit
signal, or with the behavior of the `kill` builtin in a shell like bash.

Signed-off-by: Filipe Brandenburger <filbranden@google.com>

skill.c

@@ -493,6 +493,9 @@ static void __attribute__ ((__noreturn__))
 	argc -= optind;
 	argv += optind;
 
+	if (argc < 1)
+		kill_usage(stderr);
+
 	for (i = 0; i < argc; i++) {
 		pid = strtol_or_err(argv[i], _("failed to parse argument"));
 		if (!kill((pid_t) pid, signo))