- Calc: make the complete "what encoding to use" decision before
         calling the connectivity driver, so that the driver has
	 no ambiguity about whether it should override our setting
	 or not.

	 To this end, factorise the part of the driver that reads
	 the encoding from the file header into dbtools.

 - Calc: don't ask for encoding when the file's header give the encoding.

 - don't confuse CP850 (the default) and "don't know", including:
   * don't ignore CP850 user setting
   * don't overwrite user setting with CP850

Thanks to Julien Nabet for the extensive collaboration on this.

Change-Id: Id80b7c505858b88f717b0ce6bd890527909e5fd1