Teach keepalives to exit when their parent dies

keepalive processes are typically killed by the main shell process.
However if the main shell exits the keepalive may linger. In WSL
keepalives are used more often, and the lingering keepalives are both
leaks and prevent the tests from finishing.

Have keepalives poll for their parent process ID and exit when it
changes, so they can clean themselves up. The polling frequency can be
low.

src/postfork.cpp

@@ -546,3 +546,15 @@ bool do_builtin_io(const char *out, size_t outlen, const char *err, size_t errle
     errno = saved_errno;
     return success;
 }
+
+void run_as_keepalive(pid_t parent_pid) {
+    // Run this process as a keepalive. In typical usage the parent process will kill us. However
+    // this may not happen if the parent process exits abruptly, either via kill or exec. What we do
+    // is poll our ppid() and exit when it differs from parent_pid. We can afford to do this with
+    // low frequency because in the great majority of cases, fish will kill(9) us.
+    for (;;) {
+        // Note sleep is async-safe.
+        if (sleep(1)) break;
+        if (getppid() != parent_pid) break;
+    }
+}