generate documentation keywords table automatically

The SQL keywords table in the documentation had until now been generated
by me every year by some ad hoc scripting outside the source tree once
for each major release.  This patch changes it to an automated process.

We have the PostgreSQL keywords available in a parseable format in
parser/kwlist.h[*].  For the relevant SQL standard versions, keep the
keyword lists in new text files.  A new script
generate-keywords-table.pl pulls it all together and produces a DocBook
table.

The final output in the documentation should be identical after this
change.

(Updates for SQL:2016 to come.)

-- 
Peter Eisentraut              http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services