Convert bcast code in slurmd to use gids from the sbcast_cred.

Requires a bit of shuttling around to pass the sbcast_cred_arg_t
to where it's needed. Adds additional checks that someone hasn't
managed to steal an sbcast credential somehow by comparing uid/gid
between auth header and the cred.

Remove some stray xfree(gids) calls that cause a double-free; before
the gids were fetched only for this call path, but now they're
handled as part of the cred.