Check for META and DCH consistency in autoconf.

If a debian/ packaging overlay exists, then the software versions defined in
the META and debian/changelog files must be the same to ensure that DKMS uses
correct paths and that users see the same version everywhere.
Move META handling from debian/rules to autoconf.

Let the config/zfs-meta.m4 script ensure version consistency between the META
and debian/changelog files. This change depends on an earlier commit in the
patch-queue branch:  Check-for-META-and-DCH-consistency-in-autoconf.patch

Modifying the META file at build time could cause dpkg-source to fail on an
automatic patch when the recleaning rules glitched. This would randomly
frustrate people trying to recompile the package in a non-pristine build area.
Create a zfs-doc package for shared files.

Move /usr/share/doc/* materials into a separate package and use the
dh_installdocs --link-doc switch to reduce boilerplate redundancy.

By policy, this creates a new automatic dependency on zfs-doc for all other
packages. The administrator can further reduce installation size by enabling
the /etc/dpkg/dpkg.cfg.d/zfs-doc part.