Cygwin: fix up proc_subproc flags and matching pinfo methods

Cygwin: fix up proc_subproc flags and matching pinfo methods

After patch 23a779bf3d7c2afc9eab88f6b8727c1db5544547
"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>