SYSDB: Sanitize filter before sysdb_search_groups
    
    sysdb_delete_user fails with EIO if user does not exist and contains
    backslashes.
    ldb could not parse filter (&(objectclass=group)(ghost=usr\\\\001)),
    because ghost value was not sanitized
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2163