[Squeak5.3a][Preference] Disable 'use shared WebClient instance' to allow smooth updating on Microsoft Windows

Hi

Recently Karl Ramberg reported that upgrading an image on Microsoft
Windows only works on the second attempt.

He confirms that is is made to work if the preference
    'Use shared WebClient instance'
is disabled.

- So I think in  this case it is better to disable the preference by default.
- In addition in the comment there is no need to note that Webclient
is present because Webclient is present in trunk.

Could somebody please commit these two change to the trunk?

Kind regards
Hannes