Minor enhancement of fakeit and fix to model weighting

New patches for xspec which can be found in the usual place.

12.6.0w

This adds a new option for the fakeit command. When prompted for the exposure time and correction norm, you may now enter a 3rd argument to change the fake background exposure time. Previously this parameter was fixed by the exposure time in the original background file upon which the fake background was based. For backwards compatibility, fakeit will produce the same behavior as before if this argument is not entered. Our thanks to Francois Dufor for making this suggestion. Report added on Jan 11, 2011.

12.6.0x

When the model weighting option of the weight command is selected and applied to a spectrum with a background file, the variance ought to include the background in addition to the model. Our thanks to Giacomo Vianello for bringing this to our attention. Report added on Jan 11, 2011.