CRED_SMALLGROUPS_NB now holds the number of inlined groups in field
'cr_smallgroups'. XU_NGROUPS stays the number of groups allowed in
'struct xucred'. The first is an implementation detail, while the
second is part of a public interface. All mentions of XU_NGROUPS in the
tree have been reviewed and only those concerning the implementation
detail have been changed to use CRED_SMALLGROUPS_NB (they were all in
'kern_prot.c').
No functional change (intended).