Determine the qport in a more random way.

YQ2 has a much more precise Sys_Milliseconds() than Vanilla Quake II and
it always start at 0, not some other semirandom value. If the client is
started by `./quake2 +connect example.com" or all user just walk their
way to the menu there's a very high propability that two ore more
clients end up with the same qport... We can't use rand(), because we're
always starting with the same seed, so all clients generate more or less
the same random numbers and we end up in the same situation.

So just call time(). It's portable and more or less in line what the
original code did for Windows. It may be necessary to implement some
kind of fallback logic just in case that still two clients end up with
the same qport, but that's a task for another day.

Closes #537.

src/common/netchan.c

@@ -24,6 +24,8 @@
  * =======================================================================
  */
 
+#include <time.h>
+
 #include "header/common.h"
 
 /*
@@ -94,8 +96,27 @@ Netchan_Init(void)
 {
 	int port;
 
-	/* pick a port value that should be nice and random */
-	port = Sys_Milliseconds() & 0xffff;
+	/* This is a little bit fishy:
+
+	   The original code used Sys_Milliseconds() as base. It worked
+	   because the original Sys_Milliseconds included some amount of
+	   random data (Windows) or was dependend on seconds since epoche
+	   (Unix). Our Sys_Milliseconds() always starts at 0, so there's a
+	   very high propability - nearly 100 percent for something like
+	   `./quake2 +connect example.com - that two or more clients end up
+	   with the same qport.
+
+	   We can't use rand() because we're always starting with the same
+	   seed. So right after client start we'll nearly always get the
+	   same random numbers. Again there's a high propability that two or
+	   more clients end up with the same qport.
+
+	   Just calling time() should be portable and is more less what
+	   Windows did in the original code. There's still a rather small
+	   propability that two clients end up with the same qport, but
+	   that needs to fixed somewhere else with some kind of fallback
+	   logic. */
+	port = time(NULL) & 0xffff;
 
 	showpackets = Cvar_Get("showpackets", "0", 0);
 	showdrop = Cvar_Get("showdrop", "0", 0);