Small code cleanup
database.hpp: Fix invalid include guard define.
database.hpp: Export set_t typedef from select statement
database-entities.hpp: More backend-store tuple handling functions and typedefs, also columns() and table_name() for each relation multientity, to support dbarray tuple-to-xmlrender autoconversion.
database-time.hpp: new datetime_impl ctor, create datetime using numeric hour/minutes/seconds/day/month/year
html_page.hpp: render_dbarray(), for automagic converting database select statements to xmlrender values, with lazy (one row at time) initialization.
xmlrenderer_context.hpp: datetime_impl specialization for xml render value, format() uses strftime formatting, output() uses strftime(%c)