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

Log Message:
-----------
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)