RFC: Make ~/.mailcap override other settings

[Lars Ingebrigtsen]

Since, well, forever, Emacs has had several sources for information
about how to display files, like /etc/mailcap, ~/.mailcap and the
`mailcap-mime-data' variable.

But Emacs' handling of priorities here has never felt quite right, and
they have historical reasons: In olden days, some distributions put a
copy of /etc/mailcap into ~/.mailcap, so you didn't really know what the
system preferences were and what the user's preferences were.

So mailcap.el used the following precedence algorithm:

1) If it's in `mailcap-mime-data', use that.

2) If a specific type, say, "image/gif" is in both /etc/mailcap and
~/.mailcap, make the ~/.mailcap version win.

3) If we have less specific types covering the same type, make the most
specific win.  So if /etc/mailcap has "image/gif; xv" and ~/.mailcap has
"image/*; feh", then "image/gif" wins because it's more specific.

I don't really think this ever made much sense.  I think if the user has
put "image/*; feh" into ~/.mailcap, then that should win.

So I propose to make this change on master.  It's backward-incompatible,
but will lead to a lot less confusion in the years ahead.

What do all y'all think?

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no