dbus-service: make sure all clients are registered before handling requests

To ensure this, we install a filter on our DBus connection, and remove
the call to fetch the caller credentials, since we don't use them (and
we wouldn't want to do that from the message processing worker thread).
dbus-environment: plug a memory leak
dbus-session: don't forget to assign signal registration ID
dbus: cleanup error handling
build: don't put generated files in $(srcdir)
dbus: plug some memory leaks
dbus-error: check for error != NULL

It's a programmer error to call this without a GError.
dbus-objects: return correct DBus error codes
dbus-error: fix error message