pgsql-server: Use case-insensitive comparison so that explicitly setting

Log Message:
-----------
Use case-insensitive comparison so that explicitly setting timezone=unknown
in postgresql.conf does the right thing. variable.c got this right, but
not pgtz.c ...

Modified Files:
--------------
pgsql-server/src/timezone:
pgtz.c (r1.11 -> r1.12)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/timezone/pgtz.c.diff?r1=1.11&r2=1.12)