fix: usr: Fix dig issues

When used with the ``+keepopen`` option with a TCP connection, iscman:`dig`
could terminate unexpectedly in rare situations. Additionally, iscman:`dig`
could hang and fail to shutdown properly when interrupted during a query.
These have been fixed.

Closes #5381

Merge branch '5381-dig-keepalive-crash' into 'main'

See merge request isc-projects/bind9!10681