template function "marshall".

ackowledge of a request for a VID from RTCPD.  Still a long, long way to go,
but it's a start.

Marshaller code can now safely unmarshall strings.  Unlike the orginal C code, the new string unmarshaling code checks both the source and destination buffers.  The original C code did not check the source buffer and a server could crash on reception of a message containing a string with no terminator character.