[9.20] fix: nil: Extend named-rrchecker multi-line parsing support

named-rrchecker now parses the braces which support multi-line input
from the beginning of the input rather than only when reading the
data fields of the record.

Closes #5336

Backport of MR !10521

Merge branch 'backport-5336-extend-named-rrchecker-multiline-support-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!10546