Net: Store an acceptor PID alongside the origin PID in a socket

* The origin PID is the PID of the process that created this socket,
  either explicitly by calling socket(), or implicitly by accepting
  a TCP connection. Note that accepting a local socket connection
  does not create a new socket, it reuses the one connect() was
  called on, so for accepted local sockets the origin PID points
  to the connecting process.

* The acceptor PID is the PID of the process that accept()ed this
  socket. For accepted TCP sockets, this is the same as origin PID.

Kernel/Net/Socket.cpp

@@ -63,6 +63,7 @@ RefPtr<Socket> Socket::accept()
 #endif
     auto client = m_pending.take_first();
     ASSERT(!client->is_connected());
+    client->m_acceptor_pid = m_origin_pid;
     client->set_setup_state(SetupState::Completed);
     client->m_connected = true;
     client->m_role = Role::Accepted;

Kernel/Net/Socket.h

@@ -80,6 +80,7 @@ public:
     KResult getsockopt(int level, int option, void*, socklen_t*);
 
     pid_t origin_pid() const { return m_origin_pid; }
+    pid_t acceptor_pid() const { return m_acceptor_pid; }
 
     timeval receive_deadline() const { return m_receive_deadline; }
     timeval send_deadline() const { return m_send_deadline; }
@@ -111,6 +112,7 @@ private:
 
     Lock m_lock { "Socket" };
     pid_t m_origin_pid { 0 };
+    pid_t m_acceptor_pid { 0 };
     int m_domain { 0 };
     int m_type { 0 };
     int m_protocol { 0 };