samba-tool: add --decrypt-samba-gpg support to 'user getpasswords' and 'user syncpass...

samba-tool: add --decrypt-samba-gpg support to 'user getpasswords' and 'user syncpasswords'

This get's the cleartext passwords by decrypting
the 'Primary:SambaGPG' value in order to provide the
virtual attributes: virtualClearTextUTF16, virtualClearTextUTF8,
virtualCryptSHA256, virtualCryptSHA512, virtualSSHA

The virtual attribute virtualSambaGPG provides the raw
(encrypted) value of the 'Primary:SambaGPG' value.

See the "password hash gpg key ids" option for the encryption part
of this feature.

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