metadata: accept EOPNOTSUPP for lchmod()

metadata: accept EOPNOTSUPP for lchmod()

On some systems (e.g. mine, Fedora 33) python2 has
(started to have?) lchmod(), as glibc has it, but
that always only returns EOPNOTSUPP if you really
try to operate on a symlink. Allow for that and
handle it just like ENOSYS.

Note that the handling of ENOSYS is wrong, since
integers can't be thrown or caught - pylint found
this as well, but since I'm fixing EOPNOTSUPP here
I need to fix that at the same time.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>