auth/munge - fix error with multiple-slurmd when credentials are replayed.

EMUNGE_CRED_REPLAYED is expected, and we should still mark the credential
as verified.

The bug here is that g_slurm_auth_verify was still returning SLURM_SUCCESS,
while the credential internally had verify = false, resulting in
g_slurm_auth_get_*() hitting an assertion failure.

Bug 6676.