extdom: replace winbind calls with POSIX/SSSD calls
    
    With the new ipa_server_mode SSSD is able to read user and group data
    from trusted AD domains directly and makes this data available via the
    NSS responder. With this mode enabled winbind is not needed anymore to
    lookup users and groups of trusted domains.
    
    This patch removed the calls to winbind from the extdom plugin and
    replaces them with standard POSIX calls like getpwnam() and calls from
    libsss_nss_idmap to lookup SIDs.
    
    Fixes https://fedorahosted.org/freeipa/ticket/3637 because now the
    extdom plugin does not need to handle idranges anymore, but everything
    is done inside SSSD.