[PATCH] Comm Write cleanups

Updated version of earlier patch. This one works again after the client
write pools merged.

* creates namespace Comm::IO for the read and write functionality to exist.

* The comm_write() functions are moved into that scope as
Comm::IO::Write() and only accept AsyncCall now.

* commio_* functions are all moved to methods of a new
Comm::Io::Callback object. Which represents either a read or a write
callback event waiting to happen. Old wrapper versions of the functions
have been removed.

* The fdc_table of pending read and write callbacks has been moved into
the Comm::IO scope with (the name iocb_table) and should be considered
private. For now the COMMIO_*_CB() macros are retained to produce a
pointer to a callback object in this table.

* libcomm-listener.la has been renamed to libcomm.la

This is not expected to produce any functionality changes.

Amos