r84: Implement --required-membership-of=, an ntlm_auth option that restricts

r84: Implement --required-membership-of=, an ntlm_auth option that restricts
all authentication to members of this particular group.

Also implement an option to allow ntlm_auth to get 'squashed' error codes,
which are safer to communicate to remote network clients.

Andrew Bartlett
(This used to be commit eb1c1b5eb086f49a230142ad2de45dc0e9691df3)