pgsql: write_nondefault_variables must take care to write

Log Message:
-----------
write_nondefault_variables must take care to write custom_variable_classes
first; otherwise backends reading the file might reject values of custom
variables.  Per experimentation with auto_explain.

Modified Files:
--------------
    pgsql/src/backend/utils/misc:
        guc.c (r1.485 -> r1.486)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c?r1=1.485&r2=1.486)