FreeSWITCH-FS-7269

ss7: adding iam_fwd_ind_HEX channel variable to set forward indicator
     IE's heximal value.
     - add <action application="export" data="freetdm_iam_fwd_ind_HEX=2301"/>
       to dialplan with expected hex value. If the outgoing ftdm channel's
       forward indicator needs to be changed, "export" needs to be put in the
       incoming channel's dialplan to set this variable value accordingly.
     - this implementation takes bits of A, CB, D, E, F, HG, I from the hex
       value. Bits of KJ, L, P-M are not taken from the hex.
     - How to calculate hex value with wanted bits:
       . the hex value is H-A-P-I, H is the highest bit to A, and next is P-I.
         I is the lowest bit in the whole field, and H is the highest bit in
         the whole field. Refer to Q.763 chapter 3.23.
       . use a text pad to fill in the bits with 0 and 1. eventually fill all
         the 16 bits
       . copy the binary value into a calculator and convert it to hex