Add handshake/stream encryption RPC commands

Replace protocol.encryption.set bitmask flags with
protocol.encryption.handshake.set and protocol.encryption.stream.set.
Add encryption_config to map commands to libtorrent EncryptionPolicy.
Startup default matches the old preset (allow_incoming, enable_retry,
prefer_plaintext → handshake=allow, stream=allow).

Update docs and examples. Peer list distinguishes RC4 (R/L),
handshake-only (H/h), and plain (r/l). Fix ui encrypted column
conditional. Add encryption config tests; remove OPTION_ENCRYPTION
parse tests. Drop the encryption command redirect.

doc/rtorrent.rc.lua-example

@@ -72,7 +72,8 @@ rc.throttle.min_peers.seed = 30
 rc.throttle.max_peers.seed = 80
 rc.trackers.numwant = 80
 
-rc.protocol.encryption.set('allow_incoming', 'try_outgoing', 'enable_retry')
+rc.protocol.encryption.handshake.set('allow')
+rc.protocol.encryption.stream.set('prefer')
 
 -- Limits for file handle resources, this is optimized for
 -- an `ulimit` of 1024 (a common default). You MUST leave