Direct syscalls using unix.Syscall(unix.SYS_*, ...) are discouraged on
macOS and might not be supported in future versions. Switch to use
unix.Ioctl{Get,Set}IfreqMTU to get and set an interface's MTU.
Signed-off-by: Tobias Klauser <[email protected]>
Signed-off-by: Jason A. Donenfeld <[email protected]>