email: outbound mail subsystem and RESETPASS

Adds a config-driven mailer: a SendEmail action the link layer pipes to
a configured command (sendmail -t, msmtp, ...) off-thread. NickServ
RESETPASS <account> emails a short-lived code to the address on file;
RESETPASS <account> <code> <newpassword> completes the reset. Codes are
node-local; the password change itself federates.

modules/protocol/inspircd.rs

@@ -281,7 +281,7 @@ impl Protocol for InspIrcd {
             }
             NetAction::Raw(s) => vec![s.clone()],
             // Internal: the link layer handles these before serialization.
-            NetAction::DeferRegister { .. } | NetAction::DeferPassword { .. } => vec![],
+            NetAction::DeferRegister { .. } | NetAction::DeferPassword { .. } | NetAction::SendEmail { .. } => vec![],
         }
     }