fix other 2 cases of conflicting destdir files

fix other 2 cases of conflicting destdir files

Cleanly fixed case where destdir file failed to be written because there
was a directory with the same name. This can be detected with no extra
system calls, and dealt with by finding all pages that wrote files
inside the directory, and removing them and the directory.

The other, inverse case would be expensive to detect in will_render,
since it would need to check each parent directory of the file to see
if the directory is really a conflicting file. But prep_writefile
already does a similar scan for symlinks in the path, so I added code
there to remove the conflicting file. This fix assumes that the file
is written using writefile, and not some other means (but using other means
would be a security hole too, so hopefully nothing does).