Let get_trust_pw() determine the machine_account_name to use.

Let get_trust_pw() determine the machine_account_name to use.

Up to now each caller used its own logic.

This eliminates code paths where there was a special treatment
of the following situation: the domain given is not our workgroup
(i.e. our own domain) and we are not a DC (i.e. it is not a typical
trusted domain situation). In situation the given domain name was
previously used as the machine account name, resulting in an account
name of DOMAIN\\DOMAIN$, which does not seem very reasonable to me.
get_trust_pw would not have obtained a password in this situation
anyways.

I hope I have not missed an important point here!

Michael