MonitorFdHup: replace pthread_cancel trick with a notification pipe

On https://github.com/NixOS/nix/issues/8946, we faced a surprising
behaviour wrt. exception when using pthread_cancel. In a nutshell when
a thread is inside a catch block and it's getting pthread_cancel by
another one, then the original exception is bubbled up and crashes the
process.

We now poll on the notification pipe from the thread and exit when the
main thread closes its end. This solution does not exhibit surprising
behaviour wrt. exceptions.

Co-authored-by: Mic92 <joerg@thalheim.io>

Fixes https://github.com/NixOS/nix/issues/8946

See also Lix https://gerrit.lix.systems/c/lix/+/1605 which is very
similar by coincidence. Pulled a comment from that.

src/libutil-tests/monitorfdhup.cc

@@ -0,0 +1,18 @@
+#include "util.hh"
+#include "monitor-fd.hh"
+
+#include <sys/file.h>
+#include <gtest/gtest.h>
+
+namespace nix {
+TEST(MonitorFdHup, shouldNotBlock)
+{
+    Pipe p;
+    p.create();
+    {
+        // when monitor gets destroyed it should cancel the
+        // background thread and do not block
+        MonitorFdHup monitor(p.readSide.get());
+    }
+}
+}