Introduce interprocess signaling on Windows

This change gets rsync working without any warning or errors. On Windows
we now create a bunch of C:\var\sig\x\y.pid shared memory files, so sigs
can be delivered between processes. WinMain() creates this file when the
process starts. If the program links signaling system calls then we make
a thread at startup too, which allows asynchronous delivery each quantum
and cancelation points can spot these signals potentially faster on wait

See #1240

test/posix/interprocess_signaling_test.c

@@ -0,0 +1,67 @@
+// Copyright 2024 Justine Alexandra Roberts Tunney
+//
+// Permission to use, copy, modify, and/or distribute this software for
+// any purpose with or without fee is hereby granted, provided that the
+// above copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL
+// WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED
+// WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE
+// AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL
+// DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR
+// PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER
+// TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+// PERFORMANCE OF THIS SOFTWARE.
+
+#include <signal.h>
+#include <stdatomic.h>
+#include <sys/mman.h>
+#include <unistd.h>
+
+atomic_int *got;
+
+void onsig(int sig) {
+  *got = sig;
+}
+
+int main(int argc, char *argv[]) {
+
+  // create process shared memory
+  got = mmap(0, 4, PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, -1, 0);
+  if (got == MAP_FAILED)
+    return 5;
+
+  // listen for signal
+  if (signal(SIGUSR1, onsig))
+    return 6;
+
+  // block signals
+  sigset_t full;
+  if (sigfillset(&full))
+    return 7;
+  if (sigprocmask(SIG_BLOCK, &full, 0))
+    return 8;
+
+  // create child process
+  int pid;
+  if (!(pid = fork())) {
+    sigset_t empty;
+    sigemptyset(&empty);
+    sigsuspend(&empty);
+    *got |= 128;
+    _exit(0);
+  }
+
+  // send signal
+  if (kill(pid, SIGUSR1))
+    return 9;
+
+  // wait for child to die
+  int ws;
+  if (wait(&ws) != pid)
+    return 10;
+  if (ws)
+    return 11;
+  if (*got != (128 | SIGUSR1))
+    return 12;
+}