process-spec: add another flag FORK_WAIT to safe_fork()

This new flag will cause safe_fork() to wait for the forked off child
before returning. This allows us to unify a number of cases where we
immediately wait on the forked off child, witout running any code in the
parent after the fork, and without direct interest in the precise exit
status of the process, except recgonizing EXIT_SUCCESS vs everything
else.

src/basic/exec-util.c

@@ -189,10 +189,9 @@ int execute_directories(
                 void* const callback_args[_STDOUT_CONSUME_MAX],
                 char *argv[]) {
 
-        pid_t executor_pid;
-        char *name;
         char **dirs = (char**) directories;
         _cleanup_close_ int fd = -1;
+        char *name;
         int r;
 
         assert(!strv_isempty(dirs));
@@ -215,7 +214,7 @@ int execute_directories(
          * them to finish. Optionally a timeout is applied. If a file with the same name
          * exists in more than one directory, the earliest one wins. */
 
-        r = safe_fork("(sd-executor)", FORK_RESET_SIGNALS|FORK_DEATHSIG|FORK_LOG, &executor_pid);
+        r = safe_fork("(sd-executor)", FORK_RESET_SIGNALS|FORK_DEATHSIG|FORK_LOG|FORK_WAIT, NULL);
         if (r < 0)
                 return r;
         if (r == 0) {
@@ -223,12 +222,6 @@ int execute_directories(
                 _exit(r < 0 ? EXIT_FAILURE : EXIT_SUCCESS);
         }
 
-        r = wait_for_terminate_and_check(name, executor_pid, WAIT_LOG);
-        if (r < 0)
-                return r;
-        if (r > 0) /* non-zero return code from child */
-                return -EREMOTEIO;
-
         if (!callbacks)
                 return 0;