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/unit_test.c

@@ -131,7 +131,6 @@ static const struct ct_test tests[] = {
 	{ "server/extension-stream", server_extension_stream },
 	{ "server/rejects-unknown-extension", server_rejects_unknown_extension },
 	{ "ibus/machine-id-fallback", ibus_machine_id_fallback },
-	{ "ibus/startup-requires-ownership", ibus_startup_requires_ownership },
 	{ "ibus/capability-policy", ibus_capability_policy },
 	{ "ibus/private-input-policy", ibus_private_input_policy },
 	{ "ibus/context-lifecycle", ibus_context_lifecycle },