[9.20] new: usr: implement rndc retransfer -force

A new optional argument '-force' has been added to the command channel
command 'rndc retransfer'. When it is specified, named aborts the
ongoing zone transfer (if there is one), and starts a new transfer.

Closes #2299

Backport of MR !9102

Merge branch 'backport-2299-implement-rndc-force-retransfer-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!9219