bgp: do less "put text into a buffer and then print the string".

bgp: do less "put text into a buffer and then print the string".

Convert most of the routines that stuff packet decode information into a
buffer to just directly print that information.

That way:

1) The buffer won't fill up, so you don't get the decode cut off in the
middle of words, etc.

2) If the routine runs past the end of the packet data, it'll return a
"packet truncated" indication, so the string doesn't get printed at all,
rather than just being printed up to the point of the truncation.

3) The code wasn't advancing the offset into the packet if an item is
being dissected when the buffer is full, so changing the buffer size
affected whether it would run off the end of the packet data.