Add optional poll() implementation under MG_ENABLE_POLL

This adds the ability to substitute select() with poll() on supported
unix environments when MG_ENABLE_POLL is defined. A make flag called
USE_POLL is provided as well.
Using poll() removes the limitation of FD_SETSIZE concurrent sockets,
generally 1024 on Linux environments.

src/arch_unix.h

@@ -22,7 +22,11 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#if defined(MG_ENABLE_POLL) && MG_ENABLE_POLL
+#include <poll.h>
+#else
 #include <sys/select.h>
+#endif
 #include <sys/socket.h>
 #include <sys/stat.h>
 #include <sys/time.h>