The one in ZEO/trigger.py was the good code, but we want it to live in
ZEO/zrpc/trigger.py.  This checkin makes that change.

The key checkin of ZEO/trigger.py had this checkin comment:
Fix trigger close/__del__.

The close() mechanism for an asyncore file_dispatcher is not safe to
call multiple times.  It's calling os.close() on a file descriptor
(int).  Guido observed that if you call close() twice, you could be in
trouble:  1) First close() call closes FD 6.  2) Another bit of code
opens a new file, getting FD 6.  3) Second close() call closes FD 6.
Waah!  FD 6 is some other file.

The workaround attempt here is to define a close() method on a trigger
that only closes the file descriptors the first time.

Also, make sure that both file descriptors are closed.  The previous
version only closed the read-end of the pipe.