s4:auth Change auth_generate_session_info to take an auth context

s4:auth Change auth_generate_session_info to take an auth context

The auth context was in the past only for NTLM authentication, but we
need a SAM, an event context and and loadparm context for calculating
the local groups too, so re-use that infrustructure we already have in
place.

However, to avoid problems where we may not have an auth_context (in
torture tests, for example), allow a simpler 'session_info' to be
generated, by passing this via an indirection in gensec and an
generate_session_info() function pointer in the struct auth_context.

In the smb_server (for old-style session setups) we need to change the
async context to a new 'struct sesssetup_context'.  This allows us to
use the auth_context in processing the authentication reply .

Andrew Bartlett