ibus: die where startup fails; print the Plan 9 way; drop what said nothing

ibusinit unwound its server and un-registered its atexit handler on a
failure whose only sequel was ibusthread dying anyway; now each failure
dies with its own message, and the test that existed to walk that
unwind goes with it.  Also gone: an empty watch-toggle callback where
libdbus takes nil, Maxconns as a second name for Maxclients, fprintf
and strerror where the rest of the daemon says fprint and %r, USED()
where a parameter can simply be unnamed, a second findcontext() for the
Properties branch, and emitcommit() taking a connection and path apart
from the context that has both.  Ownerpoll lives once, in dat.h.

tests/test.h

@@ -108,7 +108,6 @@ void	server_connection_ownership(struct ct*);
 void	server_extension_stream(struct ct*);
 void	server_rejects_unknown_extension(struct ct*);
 void	ibus_machine_id_fallback(struct ct*);
-void	ibus_startup_requires_ownership(struct ct*);
 void	ibus_capability_policy(struct ct*);
 void	ibus_private_input_policy(struct ct*);
 void	ibus_context_lifecycle(struct ct*);