Add cmdtab field to TETHYS_MODULE_V1 module declaration

NOTE: Normally a change to the module declaration macro would be
realized via a new macro TETHYS_MODULE_V2 and a module ABI bump or
something.
Add info_size field to u_module_info

This will allow module ABI changes beyond Tethys 1.0 to be handled
dynamically by first checking that the offset of any new fields are not
past .info_size.

Hopefully people are recompiling any third party modules for their
Tethys setup, but a change like this allows binary distribution of
Tethys modules.