[RFC] cgi value = 0 behaviour

To enable smsbox to accept '0' values in cgi fields, the interface should
change
and the behaviour will be broken, but I think this should be done before we
launch
the new stable version (1.2).

May I change the code and documentation now ?

What will be changed is :

mclass: instead of 0-4, will be 0-3 and -1 or not present for default

pid: 0 really means 0 and -1 or not present means not present in pdu

coding: 0-3 means 7bit, 8bit or ucs2 and -1 or not present means default
behaviour of kannel defining it with udh presence

compress: 0,1 forces no-compression (or decompression) or compression,
and -1 means whatever (I haven't yet done the code to compression)

validity: 0 means error (kannel can't send a message that already expired)
and
-1/not defined means smsc defaults

deferred: 0 means now, -1 or not defined means now too ;) (or when kannel
wants to send it?)

alt-dcs: 0 means 0x0*, 1 means 0xF*, -1 means let kannel do what it wants

So, it's only cgi fields introduced in 1.1.5++ versions.

What do you think ?