* log.c: (general) Generalise struct zebra_route_desc into
struct zebra_desc_table and, similar to route_types, add a
command_types table to describe Zserv protocol commands.
(route_types[]) use a macro to use designated initialisers
while avoiding tedious duplication.
(zserv_command_string) lookup string from zebra_desc_table,
similar to zebra_route_string
* zebra.h: Add declaration for zserv_command_string, adjust the
comments to reflect zebra_desc_table.