r19058: Implement "user cannot change password", and complete "user must change

r19058: Implement "user cannot change password", and complete "user must change
password at next logon" code.  The "password last set time" of zero now
means "user must change password", because that's how windows seems to
use it.  The "can change" and "must change" times are now calculated
based on the "last set" time and policies.

We use the "can change" field now to indicate that a user cannot change
a password by putting MAX_TIME_T in it (so long as "last set" time isn't
zero).  Based on this, we set the password-can-change bit in the
faked secdesc.