s3-auth Use *unix_token rather than utok in struct auth3_session_info

s3-auth Use *unix_token rather than utok in struct auth3_session_info

This brings this structure one step closer to the struct auth_session_info.

A few SMB_ASSERT calls are added in some key places to ensure that
this pointer is initialised, to make tracing any bugs here easier in
future.

NOTE: Many of the users of this structure should be reviewed, as unix
and NT access checks are mixed in a way that should just be done using
the NT ACL.  This patch has not changed this behaviour however.

Andrew Bartlett

Signed-off-by: Andrew Tridgell <tridge@samba.org>