Clean up qsort comparison function for GUC entries

Clean up qsort comparison function for GUC entries

guc_var_compare() is invoked from qsort() on an array of struct
config_generic, but the function accesses these directly as
strings (char *).  This relies on the name being the first field, so
this works.  But we can write this more clearly by using the struct
and then accessing the field through the struct.  Before the
reorganization of the GUC structs (commit a13833c35f9), the old code
was probably more convenient, but now we can write this more clearly
and correctly.

After this change, it is no longer required that the name is the first
field in struct config_generic, so remove that comment.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/2c961fa1-14f6-44a2-985c-e30b95654e8d%40eisentraut.org