pgsql: Fix SetClientEncoding() to maintain a cache of previously

Log Message:
-----------
Fix SetClientEncoding() to maintain a cache of previously selected encoding
conversion functions.  This allows transaction rollback to revert to a
previous client_encoding setting without doing fresh catalog lookups.
I believe that this explains and fixes the recent report of "failed to commit
client_encoding" failures.

This bug is present in 8.3.x, but it doesn't seem prudent to back-patch
the fix, at least not till it's had some time for field testing in HEAD.

In passing, remove SetDefaultClientEncoding(), which was used nowhere.

Modified Files:
--------------
    pgsql/src/backend/utils/mb:
        mbutils.c (r1.82 -> r1.83)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/mbutils.c?r1=1.82&r2=1.83)
    pgsql/src/include/mb:
        pg_wchar.h (r1.87 -> r1.88)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/mb/pg_wchar.h?r1=1.87&r2=1.88)