tun: Drop support for Linux 2.2.

tun: Drop support for Linux 2.2.

tun.c attempted to guess at runtime what kind of tun interface to use
by looking at uname.  Specifically, for Linux it would do
  TUN_FLAVOUR_BSD if u.release starts 2.2
  TUN_FLAVOUR_LINUX if u.release starts 2.4
and require the user to specify, otherwise.

Linux 2.6 and all later versions use TUN_FLAVOUR_LINUX.  The guessing
code is pretty nasty and Linux 2.2 is very very obsolete, so drop it.

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