Add cases for ENOBUFS and ENETDOWN. We need to make sure that we

catch all transient errors. This fixes situations where transient
error conditions such as network interfaces losing carrier signals
or the system running out of mbufs would result in the permanent
removal of forwarding syslog messages.

MFC after:	1 week

usr.sbin/syslogd/syslogd.c

@@ -1189,6 +1189,8 @@ fprintlog(struct filed *f, int flags, const char *msg)
 				logerror("sendto");
 				errno = e;
 				switch (errno) {
+				case ENOBUFS:
+				case ENETDOWN:
 				case EHOSTUNREACH:
 				case EHOSTDOWN:
 					break;