MFC r264417: realpath(): Properly fail "." or ".." components after

MFC r264417: realpath(): Properly fail "." or ".." components after
non-directories.

If realpath() is called on pathnames like "/dev/null/." or "/dev/null/..",
it should fail with [ENOTDIR]. Pathnames like "/dev/null/" already failed as
they should.

Also, put the check for non-directories after lstatting the previous
component instead of when the empty component (consecutive or trailing
slashes) is detected, saving an lstat() call and some lines of code.

PR:		kern/82980

git-svn-id: svn://svn.freebsd.org/base/stable/9@269890 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f