Implement max-len filter

To implement a max-length filter, we need a rule/policy/filter file that contains the corresponding prefixes with their maximum length. The entitydb contains a bash script for this purpose. If you run this script with the argument babeld with-deny, you will get a babeld-compatible list. The list includes rules to accept all maxlen-valid prefixes and filter all other CRXN prefixes.

This list can now be saved in the configuration file before in ip fd00::/8 le 64 ge 44 allow. It is recommended to automate this process.