pgsql: Extend the date type to support infinity and -infinity,

Log Message:
-----------
Extend the date type to support infinity and -infinity, analogously to
the timestamp types.  Turns out this doesn't even reduce the available
range of dates, since the restriction to dates that work for Julian-date
arithmetic is much tighter than the int32 range anyway.  Per a longstanding
TODO item.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        datatype.sgml (r1.229 -> r1.230)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/datatype.sgml?r1=1.229&r2=1.230)
        func.sgml (r1.449 -> r1.450)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml?r1=1.449&r2=1.450)
    pgsql/src/backend/utils/adt:
        date.c (r1.142 -> r1.143)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/date.c?r1=1.142&r2=1.143)
        xml.c (r1.78 -> r1.79)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/xml.c?r1=1.78&r2=1.79)
    pgsql/src/include/catalog:
        catversion.h (r1.497 -> r1.498)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/catversion.h?r1=1.497&r2=1.498)
        pg_proc.h (r1.519 -> r1.520)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_proc.h?r1=1.519&r2=1.520)
    pgsql/src/include/utils:
        date.h (r1.40 -> r1.41)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/date.h?r1=1.40&r2=1.41)
    pgsql/src/test/regress/expected:
        date.out (r1.7 -> r1.8)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/date.out?r1=1.7&r2=1.8)
    pgsql/src/test/regress/sql:
        date.sql (r1.4 -> r1.5)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/date.sql?r1=1.4&r2=1.5)