s3:auth: fill in info3 domain name in passwd_to_SamInfo3()

s3:auth: fill in info3 domain name in passwd_to_SamInfo3()

This is needed for make_session_info_from_username() to fill in the domain
name. The callchain goes like this:

make_session_info_from_username()
-> make_server_info_pw()
-> passwd_to_SamInfo3()

Other callers of passwd_to_SamInfo3() are also affected. A subsequent change
will let set_conn_force_user_group() user the domainname from the session_info
returned from make_session_info_from_username() to create substitution
context. Without this fix domain name would be NULL.

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>