convert rndc and the command channel to use netmgr

The netmgr is currently only used for incoming DNS queries and responses, and will need to be generalized to support other network functions (rndc/command channel, dispatch, statschannel, dig/delv, nsupdate, etc).

The first step is to convert rndc. This entails:

• add a working 'tcpconnect' function to netmgr so it can initiate TCP connections (currently it can only just accept them as a server).
• modify rndc to use this.
• modify isccc to accept netmgr connections.
• address any netmgr bugs or design deficiencies that turn up during this process.
• modify rndc command handler functions that depend on running under the taskmgr, so they are able to run in the netmgr event loop instead.