Cygwin: fix up proc_subproc flags and matching pinfo methods

After patch 23a779bf3d
"Cygwin: pinfo: stop remember doing reattach",
PROC_ADDCHILD actually just sets up a new child, mirroring
PROC_DETACHED_CHILD.  The actual attaching of the child is
performed by action PROC_REATTACH_CHILD or pinfo::reattach
respectively.

To better reflect what's going on, rename PROC_REATTACH_CHILD
to PROC_ATTACH_CHILD and rename pinfo::reattach to pinfo::attach.
For better readability change PROC_ADDCHILD to PROC_ADD_CHILD.
Fix comments accordingly.

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

winsup/cygwin/spawn.cc

@@ -869,7 +869,7 @@ child_info_spawn::worker (const char *prog_arg, const char *const *argv,
 	  postfork (child);
 	  if (mode == _P_DETACH
 	      ? !child.remember (true)
-	      : !(child.remember (false) && child.reattach ()))
+	      : !(child.remember (false) && child.attach ()))
 	    {
 	      /* FIXME: Child in strange state now */
 	      CloseHandle (pi.hProcess);