Checking waitpid res for zombie process reaping.

Fixes comment by @mathisloge in issue #447.

include/boost/process/v2/posix/default_launcher.hpp

@@ -411,7 +411,8 @@ struct default_launcher
             if (ec)
             {
                 detail::on_error(*this, executable, argv, ec, inits...);
-                do { ::waitpid(pid, nullptr, 0); } while (errno == EINTR);
+                int res = 0;
+                do { res = ::waitpid(pid, nullptr, 0); } while ((res == -1) && (errno == EINTR));
                 return basic_process<Executor>{exec};
             }
         }

include/boost/process/v2/posix/fork_and_forget_launcher.hpp

@@ -124,7 +124,8 @@ struct fork_and_forget_launcher : default_launcher
             if (ec)
             {
                 detail::on_error(*this, executable, argv, ec, inits...);
-                do { ::waitpid(pid, nullptr, 0); } while (errno == EINTR);
+                int res = 0;
+                do { res = ::waitpid(pid, nullptr, 0); } while ((res != -1)  && (errno == EINTR));
                 return basic_process<Executor>{exec};
             }
         }

include/boost/process/v2/posix/pdfork_launcher.hpp

@@ -161,7 +161,8 @@ struct pdfork_launcher : default_launcher
             if (ec)
             {
                 detail::on_error(*this, executable, argv, ec, inits...);
-                do { ::waitpid(pid, nullptr, 0); } while (errno == EINTR);
+                int res = 0;
+                do { res = ::waitpid(pid, nullptr, 0); } while ((res == -1) && (errno == EINTR));
                 return basic_process<Executor>{exec};
             }
         }

include/boost/process/v2/posix/vfork_launcher.hpp

@@ -121,7 +121,8 @@ struct vfork_launcher :  default_launcher
         if (ec)
         {
             detail::on_error(*this, executable, argv, ec, inits...);
-            do { ::waitpid(pid, nullptr, 0); } while (errno == EINTR);
+            int res = 0;
+            do { res = ::waitpid(pid, nullptr, 0); } while ((res != -1) && (errno == EINTR));
             return basic_process<Executor>{exec};
         }