Backport recastnavigation revision 254.

Fixes for:
* function comments out of date.
* dtNavMesh::init doesn't return correct dtStatus

See:
* https://code.google.com/p/recastnavigation/issues/detail?id=140
* https://code.google.com/p/recastnavigation/issues/detail?id=143
* https://code.google.com/p/recastnavigation/issues/detail?id=147