MFC rev 1.97:
The Linux unlink syscall uses a different errno value when trying to unlink
a directory.

PR: 102897 [1] http://www.FreeBSD.org/cgi/query-pr.cgi?pr=102897
Noticed by: Knut Anders Hatlen <kahatlen@gmail.com> [1], testrun with LTP
Submitted by: Marcin Cieslak <saper@SYSTEM.PL>
Tested by: netchild (LTP test run)
Approved by: re