Fix for rearranging encoding id ISO-8859-5 to ISO-8859-8.

Also make the code more robust by searching for target encoding
in the internal charset map.

Problem reported by Sagi Bashari on 2005/12/21.
See "[BUGS] BUG #2120: Crash when doing UTF8<->ISO_8859_8 encoding conversion"
on pgsql-bugs list for more details.