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.h

@@ -193,6 +193,7 @@ int exec_spawn(ExecCommand *command,
                struct CGroupBonding *cgroup_bondings,
                struct CGroupAttribute *cgroup_attributes,
                const char *cgroup_suffix,
+               int pipe_fd[2],
                pid_t *ret);
 
 void exec_command_done(ExecCommand *c);