pgsql: Arrange for timezone names to be recognized case-insensitively;

Log Message:
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Arrange for timezone names to be recognized case-insensitively; for
example SET TIME ZONE 'america/new_york' works now. This seems a good
idea on general user-friendliness grounds, and is part of the solution
to the timestamp-input parsing problems I noted recently.

Modified Files:
--------------
pgsql/doc/src/sgml:
datatype.sgml (r1.176 -> r1.177)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/datatype.sgml.diff?r1=1.176&r2=1.177)
pgsql/src/timezone:
localtime.c (r1.14 -> r1.15)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/timezone/localtime.c.diff?r1=1.14&r2=1.15)
pgtz.c (r1.46 -> r1.47)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/timezone/pgtz.c.diff?r1=1.46&r2=1.47)
pgtz.h (r1.17 -> r1.18)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/timezone/pgtz.h.diff?r1=1.17&r2=1.18)
zic.c (r1.16 -> r1.17)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/timezone/zic.c.diff?r1=1.16&r2=1.17)