pgsql: Implement max() and min() aggregates for array types.

Log Message:
-----------
Implement max() and min() aggregates for array types. Patch from Koju
Iijima, reviewed by Neil Conway. Catalog version number bumped,
regression tests updated.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        func.sgml (r1.238 -> r1.239)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml.diff?r1=1.238&r2=1.239)
    pgsql/src/backend/utils/adt:
        arrayfuncs.c (r1.115 -> r1.116)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/arrayfuncs.c.diff?r1=1.115&r2=1.116)
    pgsql/src/include/catalog:
        catversion.h (r1.256 -> r1.257)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/catversion.h.diff?r1=1.256&r2=1.257)
        pg_aggregate.h (r1.48 -> r1.49)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_aggregate.h.diff?r1=1.48&r2=1.49)
        pg_proc.h (r1.351 -> r1.352)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_proc.h.diff?r1=1.351&r2=1.352)
    pgsql/src/include/utils:
        array.h (r1.51 -> r1.52)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/array.h.diff?r1=1.51&r2=1.52)
    pgsql/src/test/regress/expected:
        arrays.out (r1.23 -> r1.24)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/arrays.out.diff?r1=1.23&r2=1.24)
    pgsql/src/test/regress/sql:
        arrays.sql (r1.18 -> r1.19)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/arrays.sql.diff?r1=1.18&r2=1.19)