make config parsing strict when starting gobgpd

[myaut]

The default policy for gobgp if config is invalid is to start with default policy ACCEPT. This is harmful for our Route Reflectors setup in which we can get spurous routes after unlucky release.

This commit adds --config-strict option that makes gobgp do the fatal exit if config is invalid. If option is not specified, gobgpd will still log warning and continue working.

It also introduces special facilty logging field similar to the one in syslog. This allows to implement said backward compatibility with Fatal() calls with facility "config" being downgraded to Warn().

Reload behavior is not changed as it is not as harmful (policies are retained in such case).

[SkalaNetworks]

Hi, I think this is an extremely useful feature. It is easy to mess up a config and end up with full tables because your filters are not working for example (happened to me). But is using the logger to dismiss the configurations the best way to go through it?

In my opinion, there should be a way to read a config file using gobgp(d) and validating the policies before enforcing them. Error handling should be handled there.