Active directory: use static configuration. Refs #2729 #2727

- Use DNS A record, to resolve domain controllers IP.
- Enhanced AdsLdapServer prop, allowing specification of PROTO and
  PORT.
- Added AdsCredentials DB default.
- Use smb/AdsLdapAccountsBranch as search base for accounts
AD added separate group LDAP source. Refs #2727
SOGo UserDataSource: use sAMAccountName attribute as UIDFieldName. Refs #2730

This maps the user and group names known to NSS through winbind,
allowing IMAP ACLs to work properly.