Replace dirname(3) by a copy that complies to POSIX.

It turns out that the path normalization that our brand new copy of
dirname(3) does is actually not allowed by the draft version of the
upcoming version of POSIX. It has to behave identically to the
dirname(1) utility.

This change replaces our new dirname(3) implementation by yet another
version that doesn't implement the path normalization logic; it merely
looks for the end of the directory name and overwrites that with a null
byte.

More details: See note #3370 at http://austingroupbugs.net/view.php?id=1073

PR: 212193
Reviewed by: emaste, jilles
Differential Revision: https://reviews.freebsd.org/D7790