Add the benchmarks directory into the main Mercury repository.

This was a seperate repository in CVS and so it missed the conversion.

benchmarks/
    As above.
Rename .cvsignore files in the benchmarks directory.

benchmarks/progs/icfp2000_par/.cvsignore:
benchmarks/progs/icfp2000_par/.gitignore:
benchmarks/progs/icfp2000_par/examples/.cvsignore:
benchmarks/progs/icfp2000_par/examples/.gitignore:
    As above.