Net: Add a basic sys$shutdown() implementation

Calling shutdown prevents further reads and/or writes on a socket.
We should do a few more things based on the type of socket, but this
initial implementation just puts the basic mechanism in place.

Work towards #428.

Kernel/Process.h

@@ -260,6 +260,7 @@ public:
     int sys$listen(int sockfd, int backlog);
     int sys$accept(int sockfd, sockaddr*, socklen_t*);
     int sys$connect(int sockfd, const sockaddr*, socklen_t);
+    int sys$shutdown(int sockfd, int how);
     ssize_t sys$sendto(const Syscall::SC_sendto_params*);
     ssize_t sys$recvfrom(const Syscall::SC_recvfrom_params*);
     int sys$getsockopt(const Syscall::SC_getsockopt_params*);