Gracefully break upgrading from earlier versions

Also remove old fixes (no longer needed) for upgrading
Don't source sage-env before sage-upgrade
Create sage-update script, split off from sage-upgrade
Source sage-env before "set -e"
Pass arguments to sage-update
Simplify the update script, error out if not fast-forward
Use the updated version for the banner, not the old one
Automatically generate the tarball=... line in checksums.ini
Fixup checksum.ini files that missed tarball=...
Trac #14715: Gracefully break upgrading from earlier versions

If older versions of Sage try to upgrade to Sage 6.0, this should break
gracefully. There should be a nice message explaining that it cannot be
done, perhaps pointing to
[http://boxen.math.washington.edu/home/release/sage-5.13/] where the
upgrade to Sage 5.13 can be done.

URL: http://trac.sagemath.org/14715
Reported by: jdemeyer
Ticket author(s): Jeroen Demeyer
Reviewer(s): Volker Braun
Remove the version argument from sage -bdist
Trac #15524: sage-fix-pkg-checksums does not preserve tarball name

This is a followup to #14968

URL: http://trac.sagemath.org/15524
Reported by: vbraun
Ticket author(s): Volker Braun
Reviewer(s):
Trac #15517: Run sage-upgrade *outside* of the Sage shell

Running `make` inside a Sage shell doesn't really work well, because
`sage-env` isn't resourced during the build. This breaks upgrading if
new packages are installed in the upgrade which require environment
variable changes. Most of the old work-arounds in `spkg/install` removed
in #14715 existed precisely for this reason.

Now that nothing can upgrade '''to''' Sage 6.0, we should make sure that
upgrading '''from''' Sage 6.0 doesn't suffer this problem.

URL: http://trac.sagemath.org/15517
Reported by: jdemeyer
Ticket author(s): Jeroen Demeyer
Reviewer(s): Volker Braun, R. Andrew Ohana
Trac #15526: banner is not updated

Starting up sage-6.0.rc0 still shows the wrong version.

URL: http://trac.sagemath.org/15526
Reported by: vbraun
Ticket author(s): Volker Braun
Reviewer(s): Volker Braun
Trac #15527: Automatically pick up version for bdist

URL: http://trac.sagemath.org/15527
Reported by: vbraun
Ticket author(s): Volker Braun
Reviewer(s): R. Andrew Ohana