scripts: plot[mpl].py: Adopted -s/--sort and -S for legend sorting

Before this, the only option for ordering the legend was by specifying
explicit -L/--add-label labels. This works for the most part, but
doesn't cover the case where you don't know the parameterization of the
input data.

And we already have -s/-S flags in other csv scripts, so it makes sense
to adopt them in plot.py/plotmpl.py to allow sorting by one or more
explicit fields.

Note that -s/-S can be combined with explicit -L/--add-labels to order
datasets with the same sort field:

  $ ./scripts/plot.py bench.csv \
          -bBLOCK_SIZE \
          -xn \
          -ybench_readed \
          -ybench_proged \
          -ybench_erased \
          --legend \
          -sBLOCK_SIZE \
          -L'*,bench_readed=bs=%(BLOCK_SIZE)s' \
          -L'*,bench_proged=' \
          -L'*,bench_erased='

---

Unfortunately this conflicted with -s/--sleep, which is a common flag in
the ascii-art scripts. This was bound to conflict with -s/--sort
eventually, so a came up with some alternatives:

- -s/--sleep -> -~/--sleep
- -S/--coalesce -> -+/--coalesce

But I'll admit I'm not the happiest about these...

scripts/codemap.py

@@ -1711,7 +1711,7 @@ if __name__ == "__main__":
             action='store_true',
             help="Pipe directly to stdout.")
     parser.add_argument(
-            '-s', '--sleep',
+            '-~', '--sleep',
             type=float,
             help="Time in seconds to sleep between redraws when running "
                 "with -k. Defaults to 2 seconds.")