Limit variable fields to 255 characters.

Limit variable fields to 255 characters.

The SIP2 protocol document specifies that variable fields are to be from
0 to 255 characters in length, not counting the field identifier or the
delimiter.

This branch adds some defensive coding to Sip::add_field to truncate any
field values that are longer than 255 characters to 255 characters before
adding them to the response.

Signed-off-by: Jason Stephenson <jstephenson@mvlc.org>