callouts: downgrade logging severity of messages to g_info() and g_debug()

callouts: downgrade logging severity of messages to g_info() and g_debug()

In dispatcher, we install a log-handler which maps G_LOG_LEVEL_MESSAGE
to syslog priority LOG_NOTICE, which in turn causes journal to highlight
the message. We don't want that so instead use g_info() and g_debug()
which maps to lower syslog levels.

There is only one problem, in debug-mode, we don't use syslog but the
default logging handler from glib. In this case, we have to set
G_MESSAGES_DEBUG otherwise g_info()/g_debug() is suppressed.