basic/exec-util: add support for synchronous (ordered) execution

The output of processes can be gathered, and passed back to the callee.
(This commit just implements the basic functionality and tests.)

After the preparation in previous commits, the change in functionality is
relatively simple. For coding convenience, alarm is prepared *before* any
children are executed, and not before. This shouldn't matter usually, since
just forking of the children should be pretty quick. One could also argue that
this is more correct, because we will also catch the case when (for whatever
reason), forking itself is slow.

Three callback functions and three levels of serialization are used:
- from individual generator processes to the generator forker
- from the forker back to the main process
- deserialization in the main process

v2:
- replace an structure with an indexed array of callbacks

src/core/manager.c

@@ -3047,7 +3047,8 @@ static int manager_run_generators(Manager *m) {
         argv[4] = NULL;
 
         RUN_WITH_UMASK(0022)
-                execute_directories((const char* const*) paths, DEFAULT_TIMEOUT_USEC, (char**) argv);
+                execute_directories((const char* const*) paths, DEFAULT_TIMEOUT_USEC,
+                                    NULL, NULL, (char**) argv);
 
 finish:
         lookup_paths_trim_generator(&m->lookup_paths);

src/core/shutdown.c

@@ -322,7 +322,7 @@ int main(int argc, char *argv[]) {
         arguments[0] = NULL;
         arguments[1] = arg_verb;
         arguments[2] = NULL;
-        execute_directories(dirs, DEFAULT_TIMEOUT_USEC, arguments);
+        execute_directories(dirs, DEFAULT_TIMEOUT_USEC, NULL, NULL, arguments);
 
         if (!in_container && !in_initrd() &&
             access("/run/initramfs/shutdown", X_OK) == 0) {