Fix the isc_nm_closedown() to actually close the pending connections

Fix the isc_nm_closedown() to actually close the pending connections

1. The isc__nm_tcp_send() and isc__nm_tcp_read() was not checking
   whether the socket was still alive and scheduling reads/sends on
   closed socket.

2. The isc_nm_read(), isc_nm_send() and isc_nm_resumeread() have been
   changed to always return the error conditions via the callbacks, so
   they always succeed.  This applies to all protocols (UDP, TCP and
   TCPDNS).