- implement r_noborder, inspired by patch from ensiform (#4289)

- Add Sys_SetEnv for portable setting of environment variables
 - Default ioquake3 to appear on the center of the screen if running in windowed mode.

code/sys/sys_unix.c

@@ -583,3 +583,19 @@ void Sys_PlatformInit( void )
 	signal( SIGIOT, Sys_SigHandler );
 	signal( SIGBUS, Sys_SigHandler );
 }
+
+/*
+==============
+Sys_SetEnv
+
+set/unset environment variables (empty value removes it)
+==============
+*/
+
+void Sys_SetEnv(const char *name, const char *value)
+{
+	if(value && *value)
+		setenv(name, value, 1);
+	else
+		unsetenv(name);
+}