Windows: Permit docbook validation using xmllint

Windows: Permit docbook validation using xmllint

Add a "check" rule to each of the docbook directories
that uses xmllint.exe (from Cygwin) to validate the
docbook source against the DTD.  Validation failures
will halt the build.

Change-Id: Ifcfcab11d5077381bfe7fa5e3767070a8d55cb21
Reviewed-on: http://gerrit.openafs.org/2391
Reviewed-by: Russ Allbery <rra@stanford.edu>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>