Cygwin: drop PROC_DETACHED_CHILD flag

Cygwin: drop PROC_DETACHED_CHILD flag

pinfo::remember with the detach parameter set to true is
the only way to call proc_subproc with PROC_DETACHED_CHILD.
This call is exclusively used in spawn to set up a pinfo for
a detached child, and that pinfo goes out of scope right
afterwards without any further action.

Drop the flag and drop the detach parameter from pinfo::remember.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>