netmgr refactoring: use generic functions when operating on sockets.
tcpdns used transport-specific functions to operate on the outer socket. Use generic ones instead, and select the proper call in netmgr.c. Make the missing functions (e.g. isc_nm_read) generic and add type-specific calls (isc__nm_tcp_read). This is the preparation for netmgr TLS layer.
(cherry picked from commit 5fedd21e)