mkdir: allow 'mkdir -p' with trailing '/'

mkdir: allow 'mkdir -p' with trailing '/'

Let's suppose that /usr/tmp exists and one wants /usr/tmp/a/b
If one runs "mkdir -p /usr/tmp/a/b/" (the '/' at the end is
important), then a "File exists" error comes up. Example:

$ rm -rf /usr/tmp/a
$ mkdir -p /usr/tmp/a/b/
/usr/tmp/a/b/: File exists

This breaks gcc47 installation when C++ is enabled, and this
isn't the behaviour of mkdir on NetBSD nor Linix.

This patch fixes the above issue by dropping the trailing '/'.