Unify MTU datatypes across the codebase

The codebase uses int and unit16 to represent MTU randomly. This change
unifies it to a MTU type from types package, which is currently uint16.

src/defaults/defaults.go

@@ -1,5 +1,7 @@
 package defaults
 
+import "github.com/yggdrasil-network/yggdrasil-go/src/types"
+
 // Defines which parameters are expected by default for configuration on a
 // specific platform. These values are populated in the relevant defaults_*.go
 // for the platform being targeted. They must be set.
@@ -14,7 +16,7 @@ type platformDefaultParameters struct {
 	DefaultMulticastInterfaces []string
 
 	// TUN/TAP
-	MaximumIfMTU  int
-	DefaultIfMTU  int
+	MaximumIfMTU  types.MTU
+	DefaultIfMTU  types.MTU
 	DefaultIfName string
 }