pgsql: Implement 4 new aggregate functions from SQL2003.

Log Message:
-----------
Implement 4 new aggregate functions from SQL2003. Specifically: var_pop(),
var_samp(), stddev_pop(), and stddev_samp(). var_samp() and stddev_samp()
are just renamings of the historical Postgres aggregates variance() and
stddev() -- the latter names have been kept for backward compatibility.
This patch includes updates for the documentation and regression tests.
The catversion has been bumped.

NB: SQL2003 requires that DISTINCT not be specified for any of these
aggregates. Per discussion on -patches, I have NOT implemented this
restriction: if the user asks for stddev(DISTINCT x), presumably they
know what they are doing.

Modified Files:
--------------
pgsql/doc/src/sgml:
func.sgml (r1.312 -> r1.313)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml.diff?r1=1.312&r2=1.313)
pgsql/src/backend/utils/adt:
float.c (r1.121 -> r1.122)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/float.c.diff?r1=1.121&r2=1.122)
numeric.c (r1.92 -> r1.93)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/numeric.c.diff?r1=1.92&r2=1.93)
pgsql/src/include/catalog:
catversion.h (r1.319 -> r1.320)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/catversion.h.diff?r1=1.319&r2=1.320)
pg_aggregate.h (r1.53 -> r1.54)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_aggregate.h.diff?r1=1.53&r2=1.54)
pg_proc.h (r1.403 -> r1.404)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_proc.h.diff?r1=1.403&r2=1.404)
pgsql/src/include/utils:
builtins.h (r1.276 -> r1.277)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/builtins.h.diff?r1=1.276&r2=1.277)
pgsql/src/test/regress/expected:
aggregates.out (r1.12 -> r1.13)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/aggregates.out.diff?r1=1.12&r2=1.13)
pgsql/src/test/regress/sql:
aggregates.sql (r1.10 -> r1.11)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/aggregates.sql.diff?r1=1.10&r2=1.11)