Add support for discarding raw trace output or writing to /dev/null

When evaluating instrumentation overhead or measuring write I/O performance with drmemtrace in offline mode, users cannot redirect trace output to `/dev/null` or discard trace data in-memory because `-outdir` expects a standard directory path and tracer initialization creates subdirectories and opens trace files, which fail on special character devices like `/dev/null`.
Furthermore, passing `none` or an empty string to `-outdir` is rejected by the option parsing, forcing offline tracing runs to generate actual files and directories even when only the raw instrumentation overhead needs to be evaluated.
We should extend `-outdir` to accept `none` (no trace file generated) and `/dev/null` (traces generated but discarded) to allow for more precise overhead measurements.