service: introduce Type=idle and use it for gettys

Type=idle is much like Type=simple, however between the fork() and the
exec() in the child we wait until PID 1 informs us that no jobs are
left.

This is mostly a cosmetic fix to make gettys appear only after all boot
output is finished and complete.

Note that this does not impact the normal job logic as we do not delay
the completion of any jobs. We just delay the invocation of the actual
binary, and only for services that otherwise would be of Type=simple.

src/core/execute.c

@@ -37,6 +37,7 @@
 #include <sys/mount.h>
 #include <linux/fs.h>
 #include <linux/oom.h>
+#include <sys/poll.h>
 
 #ifdef HAVE_PAM
 #include <security/pam_appl.h>
@@ -963,6 +964,7 @@ int exec_spawn(ExecCommand *command,
                CGroupBonding *cgroup_bondings,
                CGroupAttribute *cgroup_attributes,
                const char *cgroup_suffix,
+               int idle_pipe[2],
                pid_t *ret) {
 
         pid_t pid;
@@ -1050,6 +1052,15 @@ int exec_spawn(ExecCommand *command,
                         goto fail_child;
                 }
 
+                if (idle_pipe) {
+                        if (idle_pipe[1] >= 0)
+                                close_nointr_nofail(idle_pipe[1]);
+                        if (idle_pipe[0] >= 0) {
+                                fd_wait_for_event(idle_pipe[0], POLLHUP, DEFAULT_TIMEOUT_USEC);
+                                close_nointr_nofail(idle_pipe[0]);
+                        }
+                }
+
                 /* Close sockets very early to make sure we don't
                  * block init reexecution because it cannot bind its
                  * sockets */