Add support for seeding other networks

[maaku]

Adds four command-line options for specifying parameters necessary to run bitcoin-seeder as a DNS seed for bitcoin-like networks other than mainnet/testnet:

• “--seed ” to override the built-in bitcoin seed nodes;
• “--p2port ” to specify the default port for the network;
• “--magic ” to specify the protocol header/magic bytes; and
• “--minheight” to specify the minimum block height a node must report.

This is sufficient to enable bitcoin-seeder to be used for seeding signet, local regtest networks for testing, or networks more distantly related to bitcoin such as Blockstream's Liquid.