Use correct signal oldset in posix_spawn implementation

Use correct signal oldset in posix_spawn implementation

posix_spawn was restoring the wrong signal set, which still had
SIGCHLD and SIGINT masked, causing problems with child processes that
spawned child processes.  (Bug#54667)

See the thread ending at
https://lists.gnu.org/archive/html/emacs-devel/2022-03/msg00067.html
for more details.

* src/callproc.c (emacs_spawn): Pass oldset parameter.
(emacs_posix_spawn_init_attributes): Use correct oldset.
(emacs_posix_spawn_init): Remove intermediate function.