This adds a --do-not-upgrade option to 'guix package'.
One issue is that if you write:

  guix package -u --do-not-upgrade icecat

then this will be interpreted as:

  guix package -u icecat --do-not-upgrade

The reason is that -u is specified as taking an optional argument, and
it will take one even if it comes after some other options.

We could fix this by specifying that -u/--upgrade does _not_ take an
optional argument, instead relying on the 'arg-handler' logic to
interpret arguments.  The downside is that this would no longer be
allowed:

  guix package --upgrade=<pattern>

instead it would have to be written without the '=', like this:

  guix package --upgrade <pattern>

What do you think?

Anyway, here is my current patch which still has the issue described
above, so you must write "guix package -u . --do-not-upgrade icecat".

      Mark