Use actual pointers in dispatch_cfg.c.

Previously, I had used integers encoded as pointers.  This
introduced a flaw: NULL represented both the integer zero, and the
absence of a setting.  This in turn made the checks in
cfg_msg_set_{type,chan}() not actually check for an altered value if
the previous value had been set to zero.

Also, I had previously kept a pointer to a dispatch_fypefns_t rather
than making a copy of it.  This meant that if the dispatch_typefns_t
were changed between defining the typefns and creating the
dispatcher, we'd get the modified version.

Found while investigating coverage in pubsub_add_{pub,sub}_()