Use a common function to create the SAM_ACCOUNT being used to add accounts

Use a common function to create the SAM_ACCOUNT being used to add accounts
to the system.  This means that we always run Get_Pwnam(), and can never add
FOO when foo exists on the system (the idea is to instead add foo into
the passdb, using it's full name, RID etc).

Andrew Bartlett
(This used to be commit bb79b127e02cefae13c822fd0fd165f1f214b740)