* io.c (interpret_seek_whence): support SEEK_DATA and SEEK_HOLE.
  These are whences for lseek(2) supported by Linux since version 3.1.
  [ruby-core:56123] [Feature #8671]

* test/ruby/test_io.rb: Add tests for above.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42211 b2dd03c8-39d4-4d8f-98ff-823fe69b080e