Fix kernel support for 2.6.23+

Kernel support was broken since 2.6.23+
This patch fixes it.
Many thanks to wl500g.googlecode.com project for idea and help.
Remove if_pppol2tp.h header

if_pppol2tp.h header is not compatible with newer kernels.
We may drop it to use /usr/include/linux one.
Enable kernel-mode by default
Actually force userspace when using SAref.

Currently only warning is printed, but value don't switch actually.
Wrap connect_pppol2tp() with ifdef

Eliminate excess function call if USE_KERNEL is disabled.
Merge pull request #9 from sigwall/kernel

Fix kernel support for 2.6.23+