client, server: configurable MTU via --mtu

ex: innernet --mtu 1400 up foobarnet

Closes #102

shared/src/types.rs

@@ -384,6 +384,10 @@ pub struct NetworkOpt {
     /// Specify a WireGuard backend to use.
     /// If not set, innernet will auto-select based on availability.
     pub backend: Backend,
+
+    #[structopt(long, default_value = "1420")]
+    /// Specify the desired MTU for your interface.
+    pub mtu: u32,
 }
 
 #[derive(Debug, Clone, Deserialize, Serialize, PartialEq)]

shared/src/wg.rs

@@ -85,7 +85,7 @@ pub fn up(
         .set_private_key(wgctrl::Key::from_base64(&private_key).unwrap())
         .apply(interface, network.backend)?;
     set_addr(interface, address)?;
-    set_up(interface, 1420)?;
+    set_up(interface, network.mtu)?;
     if !network.no_routing {
         add_route(interface, address)?;
     }