Detect and forbid path separators in names

[PhilippWendler]

As discussed in #609, BenchExec crashes if a / is present in the name of a run definition. We do not want to implement proper support for allowing this because it just creates too many problems, but we should at least fail with a nice error message instead of a crash. The same is probably true for names of <tasks> tags and the --name parameter, maybe also for other names.

[kfriedberger]

Perhaps we want to forbit even more chars from usage to allow simple handling of result files everywhere. Wildcard character * in names might break our wildcard handling. For copying files around, e.g., from Linux to Windows, some more chars might be problematic. Potentially forbidden characters are \/:*?"<>| and maybe also quotes.