The mandatory header now has a mandatory list of protocols for which the
manpage is relevant.
Most man pages already has a "PROTOCOLS" section, but this introduces a
stricter way to specify the relevant protocols.
cd2nroff verifies that at least one protocol is mentioned (which can be
`*`).
This information is not used just yet, but A) the PROTOCOLS section can
now instead get generated and get a unified wording across all manpages
and B) this allows us to more reliably filter/search for protocol
specific manpages/options.
Closes#13166
b935fd4a07