opendir(3): on success, don't clobber errno

opendir(3): on success, don't clobber errno

The opendir(3) function was setting errno to ENOTDIR even
when the directory existed and was opened successfully. This
caused git to falsely detect an error.

This change moves the errno assignment into the failure code
block. It also adds a test to test24 to check for errno
changing when opendir(3) returns success.