peer: extract check for solicited messages
peer: extract check for solicited messages

This turns peer_signal_is_solicited() into peer_message_is_solicited(),
so all our checks are combined in a single place, rather than hard-coded
in some error-path.

Signed-off-by: David Herrmann <fb53b2ddb8d141e0cb39d7c67c2f81b6bc2eb0f7@gmail.com>

bus1/dbus-broker