AddFormData: prevent only directories from being posted

Commit f4cc54cb (shipped as part of the 7.29.0 release) was a
bug fix that introduced a regression in that while trying to avoid
allowing directory names, it also forbade "special" files like character
devices and more. like "/dev/null" as was used by Oliver who reported
this regression.

Reported by: Oliver Gondža
Bug: http://curl.haxx.se/mail/archive-2013-02/0040.html