net: shell: Rename current command as net_old

This is a preparation for the refactoring. The old "net" command
is renamed "net_old" so that the individual commands can be
placed into a separate .c files.
This is done like this because we need to use the
SHELL_SUBCMD_SET_CREATE() to create the sub-command and then
use the SHELL_SUBCMD_ADD() in the .c files to add the command
into the sub-command and not get conflict with the same name
sub-command.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>