Add support for pacman (ArchLinux).

This is derived from the supermin 4 support for Arch Linux which was
added by:

- Thomas S Hatch
- Evaggelos Balaskas
- Erik Nolte
kernel: Fix typo in error message.
kernel: Fix handling of SUPERMIN_KERNEL environment variable.
tests: Hack Arch kernel location.
tests: Add distro-specific harder test.
configure: Check for cpio.

Amazingly, not in the default Arch Linux install.