ext/sockets: adding SO_BUSY_POLL socket option for Linux.

To possibly reduce the latency when polling packets by activating it
from the network interface for N microseconds.
The kernel by doing so, increase the CPU(s) activity bound to
the socket significantly.
Note that not only the kernel needs to have support enabled for it
(which the average distribution usually does) but the network
interface itself needs to support it too. If not, it is just a no-op.

Author: devnexen

ext/sockets/sockets.stub.php

@@ -454,6 +454,13 @@
  */
 const SO_EXCLBIND = UNKNOWN;
 #endif
+#ifdef SO_BUSY_POLL
+/**
+ * @var int
+ * @cvalue SO_BUSY_POLL
+ */
+const SO_BUSY_POLL = UNKNOWN;
+#endif
 #ifdef SKF_AD_OFF
 /**
  * @var int