HostServ: normalise vhosts + enforce uniqueness

Canonicalise a requested vhost the way the ircd displays it (a disallowed
underscore becomes a hyphen) before storing and applying, so what we keep
matches what shows on the network. A vhost can no longer be assigned to two
accounts: prepare_vhost normalises, validates, and checks vhost_owner at
every assignment point (SET/REQUEST/ACTIVATE/TAKE/DEFAULT), so two inputs
that collapse to the same host are caught as one.

Approach recommended by siniStar (the ns_nethost normalisation logic).

api/src/lib.rs

@@ -601,6 +601,7 @@ pub trait Store {
     fn del_vhost(&mut self, account: &str) -> Result<bool, RegError>;
     fn vhost(&self, account: &str) -> Option<VhostView>;
     fn vhosts(&self) -> Vec<VhostView>;
+    fn vhost_owner(&self, host: &str) -> Option<String>;
     fn request_vhost(&mut self, account: &str, host: &str) -> Result<(), RegError>;
     fn vhost_request_wait(&self, account: &str) -> u64;
     fn take_vhost_request(&mut self, account: &str) -> Result<Option<String>, RegError>;