These files were originally taken from the following path from the
mozilla source tree:

  modules/plugin/samples/npruntime/

The .rsrc file in the plugin bundle can be decompiled with the
following command line:

/Developer/Tools/DeRez SimplePlugin.rsrc /System/Library/Frameworks/Carbon.framework/Headers/Carbon.r -useDF

Found this technical note on plugins:

  http://www.mozilla.org/projects/plugins/plugin_scripting_ABI_technote.html

And after further finding out that the API is different on OS X than
it is on Windows, I've decided that making an NPAPI plugin is too much
of a hassle for now--its variance on different platforms, etc. may
even be why Google Gears only uses NPAPI for Safari.