execute: send handoff timestamps from executor to service manager

This changes the executor to systematically send handoff timestamps to
the service manager if a socket for that is supplied. This drops the
code that did this via Type=exec messages, and reverts that part to the
old behaviour before 93cb78aee2.

Benefits of this approach:

1. We can collect the handoff for any command we fork off, regardless
   if it's ExecStart= something else, regardless whether it's Type=exec,
   Type=simple or some any other service type, regardless of the unit
   type.

2. We collect both CLOCK_REALTIME and CLOCK_MONOTONIC, as we do for the
   other process timestamps.

3. It's entirely backwards compatible, as this doesn't change the
   protocol between service manager and executor, but just extends it.

src/core/execute-serialize.c

@@ -1373,6 +1373,10 @@ static int exec_parameters_serialize(const ExecParameters *p, const ExecContext
         if (r < 0)
                 return r;
 
+        r = serialize_fd(f, fds, "exec-parameters-handoff-timestamp-fd", p->handoff_timestamp_fd);
+        if (r < 0)
+                return r;
+
         if (c && exec_context_restrict_filesystems_set(c)) {
                 r = serialize_fd(f, fds, "exec-parameters-bpf-outer-map-fd", p->bpf_restrict_fs_map_fd);
                 if (r < 0)
@@ -1620,6 +1624,14 @@ static int exec_parameters_deserialize(ExecParameters *p, FILE *f, FDSet *fds) {
                                 continue;
 
                         p->exec_fd = fd;
+                } else if ((val = startswith(l, "exec-parameters-handoff-timestamp-fd="))) {
+                        int fd;
+
+                        fd = deserialize_fd(fds, val);
+                        if (fd < 0)
+                                continue;
+
+                        close_and_replace(p->handoff_timestamp_fd, fd);
                 } else if ((val = startswith(l, "exec-parameters-bpf-outer-map-fd="))) {
                         int fd;