Replace ArchLinux module detection with generic method.

Replace ArchLinux module detection with generic method.

ArchLinux used to keep its kernel in /boot/vmlinuz26 but, with
Linux 3.0, now uses /boot/vmlinuz-linux.  Instead of just changing the
kernel filename and module directory, this change removes the ArchLinux
specific code, lets febootstrap find a kernel, and then computes the
module directory from the version string extracted from the actual kernel
file.