pgsql: Allow assignment to array elements not contiguous with those

Log Message:
-----------
Allow assignment to array elements not contiguous with those already
present; intervening positions are filled with nulls.  This behavior
is required by SQL99 but was not implementable before 8.2 due to lack
of support for nulls in arrays.  I have only made it work for the
one-dimensional case, which is all that SQL99 requires.  It seems quite
complex to get it right in higher dimensions, and since we never allowed
extension at all in higher dimensions, I think that must count as a
future feature addition not a bug fix.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        array.sgml (r1.51 -> r1.52)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/array.sgml.diff?r1=1.51&r2=1.52)
    pgsql/src/backend/utils/adt:
        arrayfuncs.c (r1.131 -> r1.132)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/arrayfuncs.c.diff?r1=1.131&r2=1.132)
    pgsql/src/test/regress/expected:
        arrays.out (r1.31 -> r1.32)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/arrays.out.diff?r1=1.31&r2=1.32)
    pgsql/src/test/regress/sql:
        arrays.sql (r1.24 -> r1.25)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/arrays.sql.diff?r1=1.24&r2=1.25)