If mkdirp() is called with a NULL ptr for the path arg, it will return
-1 with errno unchanged.  This is unexpected since on error it should
return -1 and set errno to one of the error values listed for mkdir(2).

This commit sets errno = ENOENT for this NULL ptr case.  This is in
accordance with the errors specified by mkdir(2):

  ENOENT
  A component of the path prefix does not exist or is a null pathname.

Signed-off-by: Chris Dunlap <cdunlap@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #2248

All changes needed for the libspl layer.  This includes modifications
to files directly copied from OpenSolaris and the addition of new
files needed to fill in the gaps.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>