Kill some warnings on Ubuntu.
* src/UserCommandReader.cc (UserCommandInterface): Explicitly
ignore the return code of system() to kill a warning.
* src/TestOperations.cc (generateBuchiAutomaton): Explicitly
ignore the return code of write() to kill a warning.
and SIGQUIT.
* src/ExternalTranslator.cc (ExternalTranslator::translate): Likewise.
* src/main.cc (main): Do not intercept SIGINT in
non-interactive runs.