add netmgr functions to support outgoing DNS queries

add netmgr functions to support outgoing DNS queries

- isc_nm_tcpdnsconnect() sets up up an outgoing TCP DNS connection.
- isc_nm_tcpconnect(), _udpconnect() and _tcpdnsconnect() now take a
  timeout argument to ensure connections time out and are correctly
  cleaned up on failure.
- isc_nm_read() now supports UDP; it reads a single datagram and then
  stops until the next time it's called.
- isc_nm_cancelread() now runs asynchronously to prevent assertion
  failure if reading is interrupted by a non-network thread (e.g.
  a timeout).
- isc_nm_cancelread() can now apply to UDP sockets.
- added shim code to support UDP connection in versions of libuv
  prior to 1.27, when uv_udp_connect() was added

all these functions will be used to support outgoing queries in dig,
xfrin, dispatch, etc.