Proposed changes to release target

[david at tethera.net]

When pushing 0.6 out the door, I found the release target in Makefile
daunting.  This was partly lack of confidence/understanding on my
part: I didn't really like the idea of starting a chain of events that
I didn't fully understand that ended up copying files to the web site.
Also, it didn't quite work for me, because my git-buildpackage
configuration changed the location of the build-area.

This sequence of commits modifies the release target so that it 
does a "dry run" by default, unless REALLY_UPLOAD=yes is passed.

I considered making a separate target for the uploading part of
releasing, but getting the dependencies right seemed a bit tricky.

I guess the part that might be inconvenient from Carl's point of view
is removing the automatic building and uploading to Debian. My
reasoning for this is discussed in the relevant commit message.  I'd
be happy to have a seperate target that did this, although I
personally would not use such a target.

Summary:

 [PATCH 1/6] build-system: replace use of ssh with wget for checking
 [PATCH 2/6] build-system: remove calls to git-buildpackage and dput
 [PATCH 3/6] build-system: tag earlier in the release process.
 [PATCH 4/6] build-system: use signed tag to make tar file, if available
 [PATCH 5/6] build-system: Use existing signed tag for release, if present
 [PATCH 6/6] build-system: guard parts irrevokable parts of release