Fix a zone refresh bug in zone.c:refresh_callback()

Fix a zone refresh bug in zone.c:refresh_callback()

When the zone.c:refresh_callback() callback function is called during
a SOA request before a zone transfer, it can receive a
ISC_R_SHUTTINGDOWN result for the sent request when named is shutting
down, and in that case it just destroys the request and finishes the
ongoing transfer, without clearing the DNS_ZONEFLG_REFRESH flag of the
zone. This is alright when named is going to shutdown, but currently
the callback can get a ISC_R_SHUTTINGDOWN result also when named is
reconfigured during the ongoibg SOA request. In that case, leaving the
DNS_ZONEFLG_REFRESH flag set results in the zone never being able
to refresh again, because any new attempts will be caneled while
the flag is set. Clear the DNS_ZONEFLG_REFRESH flag on the 'exiting'
error path of the callback function.