fix: dev: Refactor to use list-like macro for message sections

In the code base it is very common to iterate over all names in a message
section and all rdatasets for each name, but various idioms are used for
iteration.

This commit standardizes them as much as possible to a single idiom,
through the macro `MSG_SECTION_FOREACH`, similar to the existing
`ISC_LIST_FOREACH`.

Merge branch 'alessio/message-namelist-refactor' into 'main'

See merge request isc-projects/bind9!10302