convert rndc and control channel to use netmgr

convert rndc and control channel to use netmgr

- updated libisccc to use netmgr events
- updated rndc to use isc_nm_tcpconnect() to establish connections
- updated control channel to use isc_nm_listentcp()

open issues:

- the control channel timeout was previously 60 seconds, but it is now
  overridden by the TCP idle timeout setting, which defaults to 30
  seconds. we should add a function that sets the timeout value for
  a specific listener socket, instead of always using the global value
  set in the netmgr. (for the moment, since 30 seconds is a reasonable
  timeout for the control channel, I'm not prioritizing this.)
- the netmgr currently has no support for UNIX-domain sockets; until
  this is addressed, it will not be possible to configure rndc to use
  them. we will need to either fix this or document the change in
  behavior.