Add FdeCbParams parameter object to CommCalls API.

Add FdeCbParams parameter object to CommCalls API.

The problem:
  CommCalls API functionality is conflated with comm operational calls
created to do general FD handling (FD as pipe handle, FD as disk handle,
FD as pointer into the fd_table structure). Sometimes because they do
operations mirroring comm handlers and also use FD.  None of this actually
requires the CommCalls layer to be involved though. The Comm::Connection
objects which CommCall TCP handlers pass around is also very inappropriate
for these FD types.

This adds FdeCbParams to CommCalls infrastructure, for use internally and
"lower" than comm API to pass around raw FD values. This should be avoided
on TCP socket FD, but may be used by callers needing FD where
Comm::Connection is inappropriate.