ENAMETOOLONG, posix_rename fix (#3869)

* ENAMETOOLONG on paths > 255 characters
Corrected posix_rename behaviour:
* handles errors only if dst exists (shouldn't error out if doesn't)
* recursively removes "old name" (would otherwise fail if it's a not-empty dir)
* actually creates target directory

* Updated detection

* destubbed unlink()

* proper error returned