Cache Dwfl's for reuse between pass 2 and pass 3.

Cache Dwfl's for reuse between pass 2 and pass 3.

* setupdwfl.h: Introduce DwflPtr.
* setupdwfl.cxx: Cache kernel_dwfl and user_dwfl. Keep track of last used
  module strings. Return cached versions if same query used.
* dwflpp.h: Use DwflPtr instead of Dwfl*.
* dwflpp.cxx: Use DwflPtr and don't dwfl_end().
* translate.cxx: Likewise. Run through dwfl_getmodules() with returned
  ptr offset.