qbfsat: Add `-solver` option and allow choice of Z3 or Yices, making Yices the default.

qbfsat: Add `-solver` option and allow choice of Z3 or Yices, making Yices the default.

Ensures that "BV" is the logic whenever solving an exists-forall problem with Yices, moves the "(set-logic ...)" directive above any non-info line, sets the `ef-max-iters` parameter to a very high number when using Yices in exists-forall mode so as not to prematurely abandon difficult problems, and does not provide the incompatible "--incremental" Yices argument when in exists-forall mode.