src: fix debug build add-on loading

Before this commit, the loader in src/unix_dgram.js only looked for the
release build add-on.  This commit makes it:

 * Fall back to the debug build if no release build is found, and

 * Removes the node-waf workaround.  Support for node-waf was removed
   in commit e7e3f71 so there is no point in keeping it around.

Fixes #5 and #9.