Allow reading from interface without additional script arguments.

Reading from an interface like `bro -i en0` no longer expects to
start reading stdin for a script to load.  Explicitly passing in
'-' as an additional command line argument still allows reading a
script from stdin.

Closes #561

src/main.cc

@@ -688,6 +688,7 @@ int main(int argc, char** argv)
 
 	if ( optind == argc &&
 	     read_files.length() == 0 && flow_files.length() == 0 &&
+	     interfaces.length() == 0 &&
 	     ! (id_name || bst_file) && ! command_line_policy )
 		add_input_file("-");