[9.18] fix: usr: Unknown directive in resolv.conf not handled properly

The line after an unknown directive in resolv.conf could accidentally be skipped, potentially affecting dig, host, nslookup, nsupdate, or delv. This has been fixed.

Closes #5084

Backport of MR !9865

Merge branch 'backport-5084-plain-unknown-keyword-in-resolv-conf-not-handled-propely-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!9878