Integrate SPL as a bundled component.

This is a refresh of the branch that was briefly published and discussed last September.  Note that it preserves the zfsonlinux/spl development history and is structured in a way to facilitate parallel maintenance.

Try it by doing this:

	# ./autogen.sh
	# ./configure
	# make
	# make install DESTDIR=/tmp/zfs-combined

The packaging has not yet been updated, but `spl.ko` should be a fully integrated build product.

From a practical perspective, this is an altogether better solution than pkg-zfs currently offers because it completely solves all of the deployment and upgrade problems involving DKMS.

From a licensing perspective, the work is incomplete.  We need to audit for GPL symbol conflicts and ask for dual-licensing privileges from a few ZoL contributors.

From a technical perspective, the only outstanding task is to whittle down the Solaris headers in `include.spl/` until only Linux headers are required to build the product.