Watch: preserve previous subscriptions.
Watch: preserve previous subscriptions.

	Fix issue #4082.

                    .------------------------------------------------------------------------------------------.
                    |               real (seconds)               |                    eff                      |
                    |--------------------------------------------+---------------------------------------------|
                    |       old        |       new       | ratio |       old        |       new        | ratio |
                    |------------------+-----------------+-------+------------------+------------------+-------|
                    | n     m      std | n     m     std |   %   | n     m      std | n     m      std |   %   |
--------------------+------------------+-----------------+-------+------------------+------------------+-------|
    benches/at-cond | 3   10.483   1.7 | 3   8.360   0.5 | -20.3 | 3   0.9984   0.1 | 3   0.9980   0.2 |  -0.0 |

	* src/runner/interpreter-visit.hxx: Do not de-register and
	re-register existing subscriptions.

aldebaran/urbi,aldebaran/urbi,urbiforge/urbi,aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,urbiforge/urbi,aldebaran/urbi,aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,urbiforge/urbi