pgsql: A client_encoding specification coming from the connection

Log Message:
-----------
A client_encoding specification coming from the connection request has
to be processed by GUC before InitPostgres, because any required lookup
of the encoding conversion function has to be done during InitializeClientEncoding.
So, I broke this last week by moving GUC processing to after InitPostgres :-(.
What we can do as a compromise is process non-SUSET variables during
command line scanning (the same as before), and postpone the processing
of only SUSET variables.  None of the SUSET variables need to be set
before InitPostgres.

Modified Files:
--------------
    pgsql/src/backend/tcop:
        postgres.c (r1.438 -> r1.439)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/postgres.c.diff?r1=1.438&r2=1.439)
    pgsql/src/backend/utils/misc:
        guc.c (r1.249 -> r1.250)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c.diff?r1=1.249&r2=1.250)
    pgsql/src/include/utils:
        guc.h (r1.56 -> r1.57)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/guc.h.diff?r1=1.56&r2=1.57)