weekend work. user / group database API.

weekend work.  user / group database API.

- split sam_passwd and smb_passwd into separate higher-order function tables

- renamed struct smb_passwd's "smb_user" to "unix_user".  added "nt_user"
plus user_rid, and added a "wrap" function in both sam_passwd and smb_passwd
password databases to fill in the blank entries that are not obtained
from whatever password database API instance is being used.

NOTE: whenever a struct smb_passwd or struct sam_passwd is used, it MUST
be initialised with pwdb_sam_init() or pwd_smb_init(), see chgpasswd.c
for the only example outside of the password database APIs i could find.

- added query_useraliases code to rpcclient.

- dealt with some nasty interdependencies involving non-smbd programs
and the password database API.  this is still not satisfactorily
resolved completelely, but it's the best i can do for now.

- #ifdef'd out some password database options so that people don't
mistakenly set them unless they recompile to _use_ those options.

lots of debugging done, it's still not finished.  the unix/NT uid/gid
and user-rid/group-rid issues are better, but not perfect.  the "BUILTIN"
domain is still missing: users cannot be added to "BUILTIN" groups yet,
as we only have an "alias" db API and a "group" db API but not "builtin-alias"
db API...
(This used to be commit 5d5d7e4de7d1514ab87b07ede629de8aa00519a1)