ddns-confgen should be an alias to tsig-keygen, not the other way around

Some releases back we modified ddns-confgen so that it would behave differently if called as tsig-keygen, and then deprecated the use of dnssec-keygen to generate TSIG keys. That's the primary use of that tool now, and it's what I think people are most likely to want to look up, but in a discussion with @sgoldlust I noticed that the manual page is still titled ddns-confgen and it's listed that way in the table of contents; tsig-keygen does not appear.

There doesn't seem to be any way to have the same man page listed twice when it documents two commands, but we should have it listed with the more useful name.

(Perhaps we could add a new man page for ddns-confgen that just links to the tsig-keygen man page, if we want them both to appear in the TOC.)