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/fork.cc

@@ -509,11 +509,11 @@ frok::parent (volatile char * volatile stack_here)
 
   /* Do not attach to the child before it has successfully initialized.
      Otherwise we may wait forever, or deliver an orphan SIGCHILD. */
-  if (!child.reattach ())
+  if (!child.attach ())
     {
       this_errno = EAGAIN;
 #ifdef DEBUGGING0
-      error ("child reattach failed");
+      error ("child attach failed");
 #endif
       goto cleanup;
     }