issue-#49: Length indicator for Router Certificate sub-option

All the length fields in CARD are 8 bits, which means restricts the size of
messages to 254 bytes. This length is insufficient for certificates.

An additional concern is that if the length field of the entire
Request/Reply mesage is the same as the sub-option length field, the
sub-option generator may want generate an option longer than would fit into
the message, since this would be allowed by the field length. The solution
for this in any case is for the sub-option generator code to respect the
overall limits on the message length, in addition to the sub-option length.

Proposal 1: Keep the length field size at 8 bits and make the units of
length be 8 octets, for a maximum 1634 bits  or 2048 bytes.

Proposal 2: Change the length field size to 16 bits, for a total of 65535
bits or 8192 bytes.

Suggested resolution: Proposal 2

Reason: Proposal 1 could result in a maximum of up to 7 padding bytes of
null data in an option or sub-option which is an extra overhead that is
unnecessary and unwanted over the air.


            jak