Enforcing "DIST_SUBDIR/DISTFILE" uniqueness

[Andrew Pantyukhin]

I'd like to propose a policy to enforce a change in
DIST_SUBDIR whenever a distfile is rerolled in-place, i.e.
when checksum changes, but name stays unchanged.

Moreover, effort should be made whenever possible to
make the old file available for download from an
alternative location.

This policy will rid us of some fetch-related headaches.
It also will make it possible to share distfiles between
hosts with ports trees of different dates. Some rare issues
might also be resolved as a result of this. For one, ftp
mirrors could be configured to allow upload, but deny
modification and/or deletion.

One thing I would personally frown upon is using
something like "fetch -o othername" to save a file with a
different name. It looks all right, but it prevents us from
looking for mirrors in an automated way when master
sites go down.