Move /src to /subprojects

This will facilitate breaking up Nix into multiple packages for each
component with Meson.

src/libutil/unix/monitor-fd.hh

@@ -1,74 +0,0 @@
-#pragma once
-///@file
-
-#include <thread>
-#include <atomic>
-
-#include <cstdlib>
-#include <poll.h>
-#include <sys/types.h>
-#include <unistd.h>
-#include <signal.h>
-
-#include "signals.hh"
-
-namespace nix {
-
-
-class MonitorFdHup
-{
-private:
-    std::thread thread;
-
-public:
-    MonitorFdHup(int fd)
-    {
-        thread = std::thread([fd]() {
-            while (true) {
-                /* Wait indefinitely until a POLLHUP occurs. */
-                struct pollfd fds[1];
-                fds[0].fd = fd;
-                /* Polling for no specific events (i.e. just waiting
-                   for an error/hangup) doesn't work on macOS
-                   anymore. So wait for read events and ignore
-                   them. */
-                fds[0].events =
-                    #ifdef __APPLE__
-                    POLLRDNORM
-                    #else
-                    0
-                    #endif
-                    ;
-                auto count = poll(fds, 1, -1);
-                if (count == -1)
-                    unreachable();
-
-                /* This shouldn't happen, but can on macOS due to a bug.
-                   See rdar://37550628.
-
-                   This may eventually need a delay or further
-                   coordination with the main thread if spinning proves
-                   too harmful.
-                */
-                if (count == 0) continue;
-                if (fds[0].revents & POLLHUP) {
-                    unix::triggerInterrupt();
-                    break;
-                }
-                /* This will only happen on macOS. We sleep a bit to
-                   avoid waking up too often if the client is sending
-                   input. */
-                sleep(1);
-            }
-        });
-    };
-
-    ~MonitorFdHup()
-    {
-        pthread_cancel(thread.native_handle());
-        thread.join();
-    }
-};
-
-
-}