Rename import of signal module because identifier signal is used as well
Rename import of signal module because identifier signal is used as well

We cannot rename the other variables named signal because some are
parameters of a public API.

ultimate-pa/benchexec,ultimate-pa/benchexec,dbeyer/benchexec,dbeyer/benchexec,sosy-lab/benchexec,ultimate-pa/benchexec,ultimate-pa/benchexec,ultimate-pa/benchexec,sosy-lab/benchexec,sosy-lab/benchexec,ultimate-pa/benchexec,dbeyer/benchexec,sosy-lab/benchexec,sosy-lab/benchexec,sosy-lab/benchexec,dbeyer/benchexec