Spoof PID across execve() on Windows

It's now possible with cosmo and redbean, to deliver a signal to a child
process after it has called execve(). However the executed program needs
to be compiled using cosmocc. The cosmo runtime WinMain() implementation
now intercepts a _COSMO_PID environment variable that's set by execve().
It ensures the child process will use the same C:\ProgramData\cosmo\sigs
file, which is where kill() will place the delivered signal. We are able
to do this on Windows even better than NetBSD, which has a bug with this

Fixes #1334

test/posix/pending_signal_execve_test.c

@@ -0,0 +1,59 @@
+// 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 <cosmo.h>
+#include <signal.h>
+#include <string.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+sig_atomic_t gotsig;
+
+void onsig(int sig) {
+  gotsig = sig;
+}
+
+int main(int argc, char* argv[]) {
+  sigset_t ss;
+  sigfillset(&ss);
+  sigprocmask(SIG_BLOCK, &ss, 0);
+  if (argc >= 2 && !strcmp(argv[1], "childe")) {
+    signal(SIGUSR1, onsig);
+    sigemptyset(&ss);
+    sigsuspend(&ss);
+    if (gotsig != SIGUSR1)
+      return 2;
+  } else {
+    int child;
+    if ((child = fork()) == -1)
+      return 2;
+    if (!child) {
+      execlp(argv[0], argv[0], "childe", NULL);
+      _Exit(127);
+    }
+    if (IsNetbsd()) {
+      // NetBSD has a bug where pending signals don't inherit across
+      // execve, even though POSIX.1 literally says you must do this
+      sleep(1);
+    }
+    if (kill(child, SIGUSR1))
+      return 3;
+    int ws;
+    if (wait(&ws) != child)
+      return 4;
+    if (ws)
+      return 5;
+  }
+}