Minor bugfixes

A couple of minor bugs that I discovered when using flac2mp3 have been fixed:
1) `flac2mp3 --help` previously returned an error because showhelp() doesn't exist; it will now call showusage()
2) When called without --preset or --lameargs, flac2mp3 would fail to properly read the default presets
3) Parallel::Forkmanager has been renamed to Parallel:ForkManager in order to match the proper casing so that it can be picked up by package managers.