Encoding:
    SILC_ASN1_OID(oid)
    SILC_ASN1_OID_T(opts, tag, oid)

    Decoding:
    SILC_ASN1_OID(&oid)
    SILC_ASN1_OID_T(opts, tag, &oid)

    Macro used to encode or decode OID string.  The OID string type
    is NULL terminated char.  Its length can be determinted with strlen().

    The `opts' is SilcAsn1Options.  The `tag' is a tag number.