[asterisk-ss7] Fix for when charge indicator is "no charge"

[Juan Carlos Castro y Castro]

Currently chan_ss7 doesn't give a cow when the caller gets a "no charge" 
in the backwards call indicator field and sends an answer frame to the 
calling channel in any case. I made this patch to fix it. It just 
records the charge indicator received in the ACM response in a field in 
the pvt struct, and when the ANM arrives, it abstains from forwarding 
the answer frame if the previously recorded value was 2 (no charge).

I tested it calling an invalid number from a SIP ATA and it works OK. I 
hear the other phone company's recording as early media and the call is 
no longer billed. I tested in 0.9, but the patch applies to 1.0.0 too.

This patch is GPL v2 and (C) Instant Solutions Ltda. of Brazil.

Juan Carlos Castro y Castro
Instant Solution Ltda. <http://www.instant.com.br/>
Rio de Janeiro - Brazil
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