Encoding:
    SILC_ASN1_BMP_STRING(str, str_len)
    SILC_ASN1_BMP_STRING_T(opts, tag, str, str_len)

    Decoding:
    SILC_ASN1_BMP_STRING(&str, &str_len)
    SILC_ASN1_BMP_STRING_T(opts, tag, &str, &str_len)

    Macro used to encode or decode BMP (UCS-2) string.  The string type is
    unsigned char and string length SilcUInt32.

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

    The string must be in UTF-8 encoding when encoding.  The decoded
    string will be in UTF-8 encoding.  The actual data is encoded to
    or decoded from BMP (UCS-2)