Moved cruft out of smb.h into ntdomain.h where it belongs. dc struct

Moved cruft out of smb.h into ntdomain.h where it belongs. dc struct
now in pipe struct (where used) rather than user_struct.
Secured machine account password changing in srv_netlog_nt.c - ensure
that only the given machine can change its own password. May need to
free this up later for NT admin tools, but this is a fail-safe secure
position for now.
Jeremy.