Switch default for result files written by benchexec

[PhilippWendler]

#1211 added new command-line arguments for choosing which result files benchexec should produce, fixing #983. However, the default remains the same and can be confusing. For the next major revision of BenchExec we should finish this:

• Choose a default mode and implement it. Likely the default mode should be either the same as --results-per-rundefinition or --results-per-rundefinition --results-per-taskset.
• Tighten rules for uniqueness and presence of names of run definitions and task sets. Add parameters for choosing which result XML files to write #1211 added such rules for cases when the new parameters are used, but in the future they should always be enforced: no mix of named and unnamed tags, and no duplicate tag names (at least whenever those names would end up in the file name of a results file).
• Get rid of the distinction between "name" and "real name" in benchexec.model.RunSet. This is no longer necessary once we have consistent names.

Basically we should go through the diff of #1211 and resolve all TODOs and all cases where new code is currently guarded behind either --results-per-rundefinition or --results-per-taskset being present.