[psg.com #151] transport: kexinit MBZ field underspecified

[" via RT" <rt+secsh%rt.psg.com@localhost>]

Survey of existing implementations conducted on the WG list suggests that 
all seem to support receiving a non-zero value, but not all support 
additional data after this field, which suggests that there may be some 
fairly tightly constrained ways to use it for extensions.