By default, the `--stderr-file` and `--stdout-file` arguments were
directing the simulator to output files named "simerr" and "simout"
respectively if an output redirect was requested.
A small annoyance is these files lack an extension meaning programs
refuse to open them, or don't do so withou additional effort. On many
systems they are assumed to scripts.
This patch adds the `.txt` extension to both, thus clearly indicating to
other programs these are text files and can be opened and read as such.
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