Add camel_network_service_can_reach_sync().

Just wrappers g_network_monitor_can_reach() using the service's own
GSocketConnectable, but also updates the "host-reachable" property.
External callers can use this to synchronously update the property,
in case they need an up-to-date result to make a decision.

Also adds equivalent asynchronous functions:

  camel_network_service_can_reach()
  camel_network_service_can_reach_finish()
CamelStore: Update "host-reachable" before connecting.
CamelFolder: Update "host-reachable" before connecting.