samba-tool: add 'user syncpasswords' command

samba-tool: add 'user syncpasswords' command

This provides an easy way to keep passwords in sync with
another account database, e.g. an OpenLDAP server.

It provides a functionality like the "passwd program"
for the "unix password sync" feature of a standalone, member
and classic (NT4) server, but for an active directory domain
controller.

The provided script is called for each account/password related
change.

Like the 'user getpassword' command it allows virtual attributes like:
virtualClearTextUTF16, virtualClearTextUTF8,
virtualCryptSHA256, virtualCryptSHA512, virtualSSHA

Note that this command should just run on a single domain controller
(typically the PDC-emulator).

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Alexander Bokovoy <ab@samba.org>