DMC-525: gfal_xrootd_rmdirG errno mapping changes

Before:
    ECANCELED => ENOTEMPTY
    ENOSYS => ENOTDIR

Now:
    EEXIST => ENOTEMPTY
    EIO => ENOTDIR

Probably introduced with xrootd 4.0.1:
* **[Client/Posix]** Provide full error mapping for POSIX interface.