fix QFileSystemEngine::createDirectory race condition

During a call to QDir::mkpath(), the same path could be created
by another process, in which case the OS mkdir will fail with EEXIST.
But the docs for mkpath() state that it's not an error if it
already exists, whereas for mkdir() it is an error.  So
QFileSystemEngine::createDirectory should accept the EEXIST error
silently if it occurs while creating the sequence of parent directories
and the final leaf directory, but should fail if EEXIST happens when
it was called from QDir::mkdir(), which is when the createParents
parameter is false.  We assume the operating system mkdir() and
CreateDirectory() are atomic, so there should be no race condition
in QDir::mkdir().  It's not necessary for mkpath() to call stat()
at each level, only to check whether an existing entry is a directory
or a file.  Also added to the autotest to verify that if the
path is an existing file, creating a dir with the same name will
fail in either mkdir or mkpath.

Task-number: QTBUG-30046
Change-Id: I926352f10654fdf3b322c8685bb85ad8b8844874
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>