src/dfinstanceosx.mm: Use m_loc_of_dfexe to find DF
src/dfinstanceosx.mm: Use m_loc_of_dfexe to find DF

... instead of trying to open /proc which doesn't exist on OSX.
fixes #119.

jarmar/Dwarf-Therapist,lethosor/Dwarf-Therapist,lethosor/Dwarf-Therapist,jarmar/Dwarf-Therapist,lethosor/Dwarf-Therapist,splintermind/Dwarf-Therapist,splintermind/Dwarf-Therapist,jarmar/Dwarf-Therapist,fricy/Dwarf-Therapist,splintermind/Dwarf-Therapist,fricy/Dwarf-Therapist,fricy/Dwarf-Therapist,lethosor/Dwarf-Therapist,splintermind/Dwarf-Therapist,fricy/Dwarf-Therapist,jarmar/Dwarf-Therapist,fricy/Dwarf-Therapist