[asterisk-ss7] new ss7 config options

[Kaloyan Kovachev]

Hello,
 i have ported Domjan's changes to trunk
(https://reviewboard.asterisk.org/r/1653/ and
https://reviewboard.asterisk.org/r/1676/), but i have some problems
documenting the new config options: ss7_use_echocontrol and sls_shift

Will appreciate if someone is using them or otherwise can shed some light
on them.

ss7_use_echocontrol - it looks as it is backwards i.e. if enabled it does
not enable echocanceler ... is the bittesting in the code wrong or the code
is fine, but the name is misleading?

ss7_default_echocontrol - this option is separate from ss7_use_echocontrol
just for indication - can't we just use ss7_use_echocontrol for the
indication too or there is an use case when we need to indicate, but not
use echocanceler?