XLALComputeFstatFromFaFb(): unified F-stat computation from {Fa,Fb,A,B,C,E}

- helper function to avoid redundant code duplication in various places
- refs #543
ComputeFstat files: pure style reformatting

- trying to stick to 'repr' style in those files:
  * function documentation at definition, not prototype
  * (mostly-)GNU style indentation and braces
  * lots-of-spaces-everywhere(TM)
- destructors: immediately return on NULL input (less hierarchy)