Fix runexec --user under Python 3.5
Fix runexec --user under Python 3.5

martin-neuhaeusser/benchexec,dbeyer/benchexec,IljaZakharov/benchexec,ultimate-pa/benchexec,ultimate-pa/benchexec,IljaZakharov/benchexec,sosy-lab/benchexec,IljaZakharov/benchexec,sosy-lab/benchexec,sosy-lab/benchexec,ultimate-pa/benchexec,martin-neuhaeusser/benchexec,ultimate-pa/benchexec,dbeyer/benchexec,martin-neuhaeusser/benchexec,IljaZakharov/benchexec,dbeyer/benchexec,ultimate-pa/benchexec,sosy-lab/benchexec,sosy-lab/benchexec,dbeyer/benchexec,martin-neuhaeusser/benchexec,sosy-lab/benchexec,ultimate-pa/benchexec