Build system: Fix check for <linux/if_tun.h> and remove our copy

 * Check for <linux/if_tun.h>, not <linux/if.h>, since the former is
   what we include.

 * Remove our embedded copy of linux/if_tun.h.  We have no need of
   this any more as all recent Linux platforms will provide this as
   part of the system.  This is good because linux/if_tun.h is
   GPLv2-only and secnet as a whole has to be GPLv3+ when built
   because it contains and depends on a few GPLv3+ things.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>