The following patches eliminate the overflows in the j2date() and date2j()

The following patches eliminate the overflows in the j2date() and date2j()
functions which limited the maximum date for a timestamp to AD 1465001.
The new limit is AD 5874897.
The files affected are:

doc/src/sgml/datatype.sgml:
    Documentation change due to patch. Included is a notice about
    the reduced range when using an eight-byte integer for timestamps.

src/backend/utils/adt/datetime.c:
    Replacement functions for j2date() and date2j() functions.

src/include/utils/datetime.h:
    Corrected a bug with the limit on the earliest possible date,
    Nov 23,-4713 has a Julian day count of -1. The earliest possible
    date should be Nov 24, -4713 with a day count of 0.

src/test/regress/expected/horology-no-DST-before-1970.out:
src/test/regress/expected/horology-solaris-1947.out:
src/test/regress/expected/horology.out:
    Copies of expected output for regression testing.
    Note: Only horology.out has been physically tested. I do not have access
    to a Solaris box and I don't know how to provoke the "pre-1970" test.

src/test/regress/sql/horology.sql:
    Added some test cases to check extended range.

John Cochran