Fix workdir iterator bugs

This fixes two bugs with the workdir iterator depth check: first
that the depth was not being decremented and second that empty
directories were counting against the depth even though a frame
was not being created for them.

This also fixes a bug with the ENOTFOUND return code for workdir
iterators when you attempt to advance_into an empty directory.
Actually, that works correctly, but it was incorrectly being
propogated into regular advance() calls in some circumstances.

Added new tests for the above that create a huge hierarchy on
the fly and try using the workdir iterator to traverse it.
Merge pull request #1411 from arrbee/workdir-iterator-depth-limit-bug

Fix workdir iterator bugs