nfscommon: Return NFSERR_ATTRNOTSUPP for AUDIT/ALARM ACEs

FreeBSD only supports Allow/Deny ACEs in NFSv4 ACLs.
As such, it does not make sense to parse Audit/Alarm
ACEs. Modify nfsrv_dissectace() so that it returns
NFSERR_ATTRNOTSUPP if an Audit/Alarm ACE is found in
the ACL being parsed. The code has been #ifdef notnow'd,
since Audit/Alarm ACEs might be supported someday.

This should not have significant impact, since FreeBSD
reports to clients that only Allow/Deny ACEs are
supported and an attempt to set one would have failed
anyhow.

MFC after: 2 weeks