Kernel: Add support for MSG_NOSIGNAL and properly send SIGPIPE

Previously we didn't send the SIGPIPE signal to processes when
sendto()/sendmsg()/etc. returned EPIPE. And now we do.

This also adds support for MSG_NOSIGNAL to suppress the signal.

Kernel/API/POSIX/sys/socket.h

@@ -60,6 +60,7 @@ extern "C" {
 #define MSG_DONTROUTE 0x10
 #define MSG_WAITALL 0x20
 #define MSG_DONTWAIT 0x40
+#define MSG_NOSIGNAL 0x80
 
 typedef uint16_t sa_family_t;