winsync-migrate: Convert entity names to posix friendly strings
    
    During the migration from winsync replicated users to their
    trusted identities, memberships are being preserved. However,
    trusted users are external and as such cannot be added as
    direct members to the IPA entities. External groups which
    encapsulate the migrated users are added as members to those
    entities instead.
    
    The name of the external group is generated from the type
    of the entity and its name. However, the entity's name can
    contain characters which are invalid for use in the group
    name.
    
    Adds a helper function to convert a given string to a string
    which would be valid for such use and leverages it in the
    winsync-migrate tool.
    
    https://fedorahosted.org/freeipa/ticket/5319
    
    Reviewed-By: Martin Babinsky <mbabinsk@redhat.com>