Fix errno used when checking for failure with `O_BENEATH` (#32)
Fix errno used when checking for failure with `O_BENEATH` (#32)

Per open(2), the errno that should be tested is `ENOTCAPABLE`, not
`EPERM`.

This addresses the first set of failures noted in #28, which account for
all failures in the openat(2) testcases, sans two in the second set of
failures.

Signed-off-by: Enji Cooper <d6bb01297bdcc8d1a5e14be12c1049fd9e385b00@gmail.com>
google/capsicum-test,google/capsicum-test,google/capsicum-test,google/capsicum-test