If requested by `b2b_build_pai_from_hdr` DSM parameter,
then get the value of PAI using this parameter and pass
it further for `B2BSipReplyEvent` processing.
Use event params to pass the value.
`b2b_build_pai_from_hdr` keeps the name of header
coming in the latest reply, from which the PAI's value
is to be built.
Update the `reinviteCaller()` function's signature,
to pass headers towards `dlg->sendRequest()`.
Change-Id: Ic6e66e326f26a830c3ed06c19a6ab7be4ebe0135