Add new JSON processing functions and parser API.

The JSON parser is converted into a recursive descent parser, and
exposed for use by other modules such as extensions. The API provides
hooks for all the significant parser event such as the beginning and end
of objects and arrays, and providing functions to handle these hooks
allows for fairly simple construction of a wide variety of JSON
processing functions. A set of new basic processing functions and
operators is also added, which use this API, including operations to
extract array elements, object fields, get the length of arrays and the
set of keys of a field, deconstruct an object into a set of key/value
pairs, and create records from JSON objects and arrays of objects.

Catalog version bumped.

Andrew Dunstan, with some documentation assistance from Merlin Moncure.
Fix page title for JSON Functions and Operators.
ecpg: Parallel make fix

In some parallel make situations, the install-headers target could be
called before the installation directories are created by installdirs,
causing the installation to fail.  Fix that by making install-headers
depend on installdirs.
Remove tab from SGML file.