Initial design of some of the functions to operate on trust passwords

Initial design of some of the functions to operate on trust passwords
from passdb backend level (tdbsam, in this case).
It is written as wrapper for secrets_ calls that use secrets.tdb file
and is not treated as eventual solution. Trust passwords are being
handled uniformly, SAM_TRUST_PASSWD structure, and so they should be
stored as well.

Note, this code is disabled ie. not used anywhere yet. I'm working
on next routines in line.

rafal
(This used to be commit 02ac9332ab1d34f47667b40ce23b2b5d04c4dff1)