Add Pow._eval_is_zero() helper

fixes #7965
Fix Pow._eval_is_negative (if base>=0) and tests:

1) if k>=0 - for k**-2 we can't say anything
   definite (i.e. it's positive), because 0**-2 is zoo.

2) Evaluate Abs(1/Abs(x)) as 1/Abs(x)

3) workaround powsimp(e, combine='base'):
   (x/y)**(-n) -> (y/x)**n for nonnegative x, y