Fix CPU usage on macOS wait_until.

Related (I believe) to boostorg#55

With the fix from klemens-morgenstern/boost-process#197, multiple processes can still at times be spawned, and they take extremely high CPU usage. This PR basically uses the same approach used for boost 1.68.0 (which was the last time wait_for seemed to work reliably) but adds a 1ms sleep to each iteration of the loop so as not to spike the CPU.

include/boost/process/detail/posix/wait_for_exit.hpp

@@ -17,6 +17,10 @@
 #include <sys/wait.h>
 #include <unistd.h>
 
+#if !defined(BOOST_POSIX_HAS_SIGTIMEDWAIT)
+#include <thread>
+#endif
+
 namespace boost { namespace process { namespace detail { namespace posix {
 
 inline void wait(const child_handle &p, int & exit_code, std::error_code &ec) noexcept
@@ -127,63 +131,18 @@ inline bool wait_until(
           (((ret == -1) && errno == EINTR) ||
            ((ret != -1) && !WIFEXITED(status) && !WIFSIGNALED(status))));
 #else
-    //if we do not have sigtimedwait, we fork off a child process  to get the signal in time
-    pid_t timeout_pid = ::fork();
-    if (timeout_pid  == -1)
-    {
-        ec = boost::process::detail::get_last_error();
-        return true;
-    }
-    else if (timeout_pid == 0)
-    {
-        auto ts = get_timespec(time_out - Clock::now());
-        ::timespec rem;
-        while (ts.tv_sec > 0 || ts.tv_nsec > 0)
-        {
-            if (::nanosleep(&ts, &rem) != 0)
-            {
-                auto err = errno;
-                if ((err == EINVAL) || (err == EFAULT))
-                    break;
-            }
-            ts = get_timespec(time_out - Clock::now());
-        }
-        ::exit(0);
-    }
-
-    struct child_cleaner_t
-    {
-        pid_t pid;
-        ~child_cleaner_t()
-        {
-            int res;
-            ::kill(pid, SIGKILL);
-            ::waitpid(pid, &res, WNOHANG);
-        }
-    };
-    child_cleaner_t child_cleaner{timeout_pid};
-
+    //if we do not have sigtimedwait, we just loop and sleep for a millisecond to prevent CPU spike
     do
     {
-        int sig_{0};
-        if ((::waitpid(timeout_pid, &status, WNOHANG) != 0)
-            && (WIFEXITED(status) || WIFSIGNALED(status)))
-
-            return false;
-
-        ret = ::sigwait(&sigset, &sig_);
         errno = 0;
 
-        if ((sig_ == SIGCHLD) &&
-            (old_sig.sa_handler != SIG_DFL) && (old_sig.sa_handler != SIG_IGN))
-            old_sig.sa_handler(ret);
-
         ret = ::waitpid(p.pid, &status, WNOHANG);
         if (ret == 0) // == > is running
         {
             timed_out = Clock::now() >= time_out;
             if (timed_out)
                 return false;
+            std::this_thread::sleep_for(std::chrono::milliseconds(1));
         }
     }
     while ((ret == 0) ||