Allow generation with non-updated filters when internet is unreachable

[RX14]

Currently, if a node is broken in a way which doesn't provide internet connectivity, it's impossible to regenerate a pathvector config which requires irr/other database lookups. This is quite frustrating when you're in a situation where a config change is required to restore connectivity.

If there was a cache of the last generated filter config when the node was online, there could be a pathvector generate option to use those filter lists when regenerating the config.

[natesales]

This sounds like a use for config snapshots! Here's how I imagine this might work:

After a successful config generation (passing BIRD config validation), store the pathvector config, the full generated output (/etc/bird/*.conf), and associated metadata (pathvector version, snapshot name/description, etc) in a config snapshot directory.

Add a few new commands to interact with snapshots (they only move files around for convenience, they don't need to do anything intelligent):

• [ ] pathvector snapshot ls (list snapshot directories and parse metadata for a nice table)
• [ ] pathvector snapshot apply [name/id] (rm /etc/bird/.conf && mv /foo/snapshots/example/.conf /etc/bird && birdc conf, then update a reference to where the current snapshot is)
• [ ] pathvector snapshot rename [name] (edit metadata file)

I'm preparing a v7 release so this might be able to make it in before then.

[RX14]

Config snapshots are nice, but on occasion I have needed to add an option that has not been in the config before to restore connectivity. This is why I suggested being able to cache filter info as a solution: it allows you any config mutation which doesn't add more filters which depend on external DBs.