Validate postgis manual using XSD and RELAXNG schemas

[strk]

In addition (or instead of) using the DTD, xmllint supports these switches:

--schema ​http://docbook.org/xml/5.0/xsd/docbook.xsd --relaxng ​http://docbook.org/xml/5.0/rng/docbook.rng

In both cases the URIs are looked up in xml catalogs, and we could use the xmlcatalog tool to verify local versions exist at configure time, this is what I have in my system:

$ xmlcatalog "" 'http://docbook.org/xml/5.0/xsd/docbook.xsd'
file:///usr/share/xml/docbook/schema/xsd/5.0/docbook.xsd
$ xmlcatalog "" 'http://docbook.org/xml/5.0/rng/docbook.rng'
file:///usr/share/xml/docbook/schema/rng/5.0/docbook.rng