pg_upgrade: make the locale comparison more tolerant

Hello pg-hackers!

I tried to look at the problem resulting in changes mentioned here:
http://www.postgresql.org/message-id/20121002155857.GE30089@momjian.us

If the system locale is changed e.g. from en_US.utf8 to en_US.utf-8 before
upgrading the data stack for newer server, pg_upgrade fails. It is quite
awkward to ask users to change locale.conf contents just for upgrade
purposes; is there other known workaround?

I attached patch aimed to remove this hurdle. After its application, the
in-place upgrade finished successfully for me under mentioned conditions,
though I am not 100% sure about its correctness.

Thanks, Pavel