Update postgresql.conf.sample to match pg_settings classificaitons.

Update postgresql.conf.sample to match pg_settings classificaitons.

A handful of settings, most notably shared_preload_libraries, were
just plain the wrong place compared to their assigned config_group
value in guc.c (and thus pg_settings).  In other cases the names of
the sections in postgresql.conf.sample were mildly different from
the corresponding entries in config_group_names[].  Make it all
consistent.

Adrián Escoms, reviewed by me.

Discussion: http://postgr.es/m/CACksPC2veEmFRYqwYepWYO9U7aFhAx6sYq+WqjTyHw7uV=E=pw@mail.gmail.com