Invoke usage for missing hostnames in -[st]

A source or destination address of the form ":1234" is parsed as a
hostname of "" which is guaranteed to never work.  While we can't
reject all possible invalid addresses at this point -- and probably
shouldn't since the resolver can give more informative errors --
this particular case can easily be produced from a broken shell
script (with a hostname variable accidentally not set), so it's more
worthy of rejection via usage() than most.

Suggested by:	Ross Richardson

spipe/main.c

@@ -221,7 +221,7 @@ main(int argc, char * argv[])
 		usage();
 	if (!(opt_o > 0.0))
 		usage();
-	if (opt_t == NULL)
+	if ((opt_t == NULL) || (opt_t[0] == ':'))
 		usage();
 
 	/* Initialize the "events & threads" cookie. */

spiped/main.c

@@ -239,9 +239,9 @@ main(int argc, char * argv[])
 		usage();
 	if ((opt_r != 60.0) && opt_R)
 		usage();
-	if (opt_s == NULL)
+	if ((opt_s == NULL) || (opt_s[0] == ':'))
 		usage();
-	if (opt_t == NULL)
+	if ((opt_t == NULL) || (opt_t[0] == ':'))
 		usage();
 
 	/*