init: introduce I2P connectivity options

Introduce two new options to reach the I2P network:

* `-i2psam=<ip:port>` point to the I2P SAM proxy. If this is set then
  the I2P network is considered reachable and we can make outgoing
  connections to I2P peers via that proxy. We listen for and accept
  incoming connections from I2P peers if the below is set in addition to
  `-i2psam=<ip:port>`

* `-i2pacceptincoming` if this is set together with `-i2psam=<ip:port>`
  then we accept incoming I2P connections via the I2P SAM proxy.

doc/files.md

@@ -64,6 +64,7 @@ Subdirectory       | File(s)               | Description
 `./`               | `ip_asn.map`          | IP addresses to Autonomous System Numbers (ASNs) mapping used for bucketing of the peers; path can be specified with the `-asmap` option
 `./`               | `mempool.dat`         | Dump of the mempool's transactions
 `./`               | `onion_v3_private_key` | Cached Tor onion service private key for `-listenonion` option
+`./`               | `i2p_private_key`     | Private key that corresponds to our I2P address. When `-i2psam=` is specified the contents of this file is used to identify ourselves for making outgoing connections to I2P peers and possibly accepting incoming ones. Automatically generated if it does not exist.
 `./`               | `peers.dat`           | Peer IP address database (custom format)
 `./`               | `settings.json`       | Read-write settings set through GUI or RPC interfaces, augmenting manual settings from [bitcoin.conf](bitcoin-conf.md). File is created automatically if read-write settings storage is not disabled with `-nosettings` option. Path can be specified with `-settings` option
 `./`               | `.cookie`             | Session RPC authentication cookie; if used, created at start and deleted on shutdown; can be specified by `-rpccookiefile` option