Expect POLLNVAL on Linux too.

When this is the default/only behaviour, need to
remember to remove the POLLNVAL_FOR_INVALID_POLLFD
conditional behaviour altogether.
Expect policing of rights outside of capability mode on Linux too.

When this is the default/only behaviour, need to
remember to remove the HAVE_RIGHTS_CHECK_OUTSIDE_CAPMODE
conditional behaviour altogether.
Use cap_rights_t as exported by kernel headers