r6024: Some of the ordering constraints on the popt callbacks were getting

r6024: Some of the ordering constraints on the popt callbacks were getting
painful, so don't call lp_*() functions until the post stage (rather
than in the cli_credentails_init(), which is called in the pre stage),
and don't open the secrets.ldb looking for the machine account details
until we actually need them (well after popt is done, and we know we have the other things right).

Set the domain and realm, as well as the account and password for -P
(fetch machine password) operation.

Allow NETLOGON credentials to be stored in this structure - will allow
SCHANNEL to be made more generic.

Clarify why we don't do special checks for NULL pointers, particularly
in the anonymous check (it indicates a programmer error, not a
run-time condition).

Also make lib/credentials.c a little more consistant.

Andrew Bartlett
(This used to be commit 730e6056b730c15008772c30cd6f7c03fb6b7e5f)