Update list of deprecated / obsolete RPM tags

[dralley]

There are a number of RPM tags that are no longer useful and are essentially deprecated which have not yet been officially marked as such in documentation or in code comments.

Some examples:

• RPMTAG_FILESIGNATURELENGTH
• 32-bit size tags, once Default to 64bit size tags in headers #864 goes through
• RPMSIGTAG_PGP, RPMSIGTAG_GPG, RPMSIGTAG_PGP5, RPMSIGTAG_MD5 if I am reading Ensure unique tags in signature header #1570 correctly

[pmatilai]

Of those, only RPMSIGTAG_PGP5 is actually obsolete. The signature length is just a stupid tag and for that the docs need to be updated.

[dralley]

Well, I did include "deprecated". I think that still applies?

IMO if a tag is "stupid and pointless" and doesn't need to be written then it is effectively deprecated and should be documented as such, yes?. My main point is to just have it documented somewhere.

There's also a bunch of tags which could be present in either header, but really ought to be in only one of the headers, and that's not very clearly documented right now either. i.e. RPMSIGTAG_* tag aliases which it seems shouldn't be used as such.

[dralley]

RPMTAG_CHANGELOG doesn't seem to be used anywhere anymore, presumably replaced by the combo (RPMTAG_CHANGELOGNAME, RPMTAG_CHANGELOGTIME, RPMTAG_CHANGELOGTEXT)