[9.20] fix: dev: Don't enable REUSEADDR on outgoing UDP sockets

The outgoing UDP sockets enabled `SO_REUSEADDR` that allows sharing of the UDP sockets, but with one big caveat - the socket that was opened the last would get all traffic.  The dispatch code would ignore the invalid responses in the dns_dispatch, but this could lead to unexpected results.

Backport of MR !9569

Merge branch 'backport-ondrej/fix-outgoing-UDP-port-selection-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!9583