feat(distributed): serve children in the search tree

The client read the distributed network but never carried it: a peer
dialling with a D connection was logged and dropped, and AcceptChildren
was hardcoded false, so this client took from the tree without ever
relaying a search for anyone else.

With accept_children set, peers hang from us: each is told where our
branch sits when it connects and again when the branch moves, every
search that reaches us is passed down, and a child that goes away loses
its slot. Ten at a time, matching Nicotine+, since each costs a socket
and a copy of the network's whole search stream — which is also why it is
opt-in, through the new accept_children config key (ignored without a
listener, since a child would have nothing to dial).

soulseek-rs-lib/src/message/distributed.rs

@@ -108,12 +108,17 @@ pub fn build_branch_level(level: i32) -> Message {
 /// What we tell the server about our place in the tree: the four messages
 /// Nicotine+ sends together whenever it changes.
 #[must_use]
-pub fn stance(root: &str, level: u32, has_parent: bool) -> Vec<Message> {
+pub fn stance(
+    root: &str,
+    level: u32,
+    has_parent: bool,
+    accept_children: bool,
+) -> Vec<Message> {
     vec![
         MessageFactory::build_have_no_parent(!has_parent),
         MessageFactory::build_branch_root(root),
         MessageFactory::build_branch_level(level),
-        MessageFactory::build_accept_children(false),
+        MessageFactory::build_accept_children(accept_children),
     ]
 }