bus-proxyd: fix EPERM on replies

Imagine a kdbus peer sending a method-call without EXPECT_REPLY set
through the proxy to a dbus1 peer. The proxy turns the missing
EXPECT_REPLY flag into a dbus1 NO_REPLY_EXPECTED flag. However, if the
receipient ignores that flag (valid dbus1 behavior) and sends a reply, the
proxy will try to forward it to the original peer. This will fail with
EPERM as the kernel didn't track the reply.

We have two options now: Either we ignore EPERM for reply messages, or we
track reply-windows in the proxy so we can properly ignore replies if
EXPECT_REPLY wasn't set.

This commit chose the first option: ignore EPERM for replies. The only
down-side is that replies without matching method call will no longer be
forwarded by the proxy. This works on dbus1, though.
Nobody sane does this, so lets ignore it.