Add -z arugment like other netcat alternatives

--- a/docs/nc6.1.in
+++ b/docs/nc6.1.in
@@ -187,6 +187,9 @@
 opposite direction to normal transfer.  If listen mode is specified, this is
 equivalent to "\-\-send-only \-\-buffer-size=65536" otherwise it is equivalent to
 "\-\-recv-only \-\-buffer-size=65536".
+.TP 13
+.I \-z
+Don't send any data (scanning mode).
 .SH UDP
 UDP support in netcat6 works very well in both connect and in listen mode.
 When using UDP in listen mode netcat6 accepts UDP packets from any source that
--- a/src/parser.c
+++ b/src/parser.c
@@ -149,6 +149,7 @@
 	int buffer_size = 0;
 	int sndbuf_size = 0;
 	int rcvbuf_size = 0;
+	bool zflag = false;
 
 	/* check arguments */
 	assert(argc > 0);
@@ -164,7 +165,7 @@
 	_verbosity_level = 0;
 
 	/* option recognition loop */
-	while ((c = getopt_long(argc, argv, "46be:T:hlnp:q:s:uvw:xXCt:",
+	while ((c = getopt_long(argc, argv, "46be:T:hlnp:q:s:uvw:xXCt:z",
 	                        long_options, &option_index)) >= 0)
 	{
  		switch (c) {
@@ -316,6 +317,9 @@
 			if (ttl < -1 || ttl > 255 )
 				fatal(_("ttl can only be set between 0 to 255"));
 			break;
+		case 'z':
+			zflag = true;
+			break;
 		case '?':
 			print_usage(stderr);
 			exit(EXIT_FAILURE);
@@ -404,6 +408,11 @@
 		fatal_internal("invalid protocol family %d", family);
 	}
 
+	/* don't set -l and -z both */
+	if (listen_mode == true && zflag == true) {
+		fatal(_("cannot set both -l and -z"));
+	}
+
 	/* set mode flags */
 	if (listen_mode == true) {
 		ca_set_flag(attrs, CA_LISTEN_MODE);
@@ -545,6 +554,9 @@
 		ca_set_sndbuf_size(attrs, sndbuf_size);
 	if (rcvbuf_size > 0)
 		ca_set_rcvbuf_size(attrs, rcvbuf_size);
+
+	/* setup zflag */
+	ca_set_zflag(attrs, zflag);
 }
 
 
@@ -640,6 +652,7 @@
 	fprintf(fp, " -X, --rev-transfer     %s\n",
 	              _("File transfer mode (reverse direction)"));
 	fprintf(fp, " -T, --ttl=TTL          %s\n", _("TTL of outgoing IP packet"));
+	fprintf(fp, " -z                     %s\n", _("Don't send any data (scan mode)"));
 	fprintf(fp, "\n");
 }
 
--- a/src/connection.h
+++ b/src/connection.h
@@ -70,6 +70,7 @@
 	bool remote_half_close_suppress;
 	bool local_half_close_suppress;
 	char *local_exec;
+	bool zflag;
 } connection_attributes_t;
 
 /* CA flags */
@@ -136,6 +137,9 @@
 #define ca_set_local_hold_timeout(CA, T)		\
 	((CA)->local_hold_timeout = (T))
 
+#define ca_zflag(CA)			((CA)->zflag)
+#define ca_set_zflag(CA, Z) 		((CA)->zflag = (Z))
+
 #define ca_remote_half_close_suppress(CA)		\
 	((CA)->remote_half_close_suppress)
 #define ca_set_remote_half_close_suppress(CA, B)	\
--- a/src/main.c
+++ b/src/main.c
@@ -174,7 +174,7 @@
 {
 	circ_buf_t remote_buffer, local_buffer;
 	io_stream_t remote_stream, local_stream;
-	int retval;
+	int retval=0;
 
 	assert(attrs != NULL);
 	assert(fd >= 0);
@@ -219,14 +219,17 @@
 			     remote_stream.mtu);
 	}
 
-	/* transfer data between endpoints */
-	retval = run_transfer(attrs, &remote_stream, &local_stream);
-
-	/* cleanup */
-	io_stream_destroy(&local_stream);
-	io_stream_destroy(&remote_stream);
-	cb_destroy(&local_buffer);
-	cb_destroy(&remote_buffer);
+	/* if not in scan mode, send data */
+	if (attrs->zflag == false) {
+		/* transfer data between endpoints */
+		retval = run_transfer(attrs, &remote_stream, &local_stream);
+
+		/* cleanup */
+		io_stream_destroy(&local_stream);
+		io_stream_destroy(&remote_stream);
+		cb_destroy(&local_buffer);
+		cb_destroy(&remote_buffer);
+	}
 
 	return retval;
 }