- move the main func declarations to nmcli.h and give them a common
prefix "nmc_command_func_" prefix.
- remove some of the header files that are now empty. In fact, these
headers did not really declare some well separated module. While we
probably should structure the code in nmcli better with better layering,
it was not and still is not. Having these dummy headers don't mean that
the code is well structured and they serve little purpose.
- move the static NMCommand lists variables into the function scope
where they are used.