plugins: Fix small typo in man pages.
src: Refactor dlopen into separate function.

This is just code motion.
Add a shorter syntax for plugin names.

You can now write:

  nbdkit gzip [args..]

instead of having to specify the plugin directory:

  nbdkit /usr/lib64/nbdkit/plugins/nbdkit-gzip-plugin.so
Add nbdkit --dump-config option to print libdir, plugindir.
Update TODO file with further ideas.
perl: New plugin lets you write plugins as Perl scripts.