samples net/posix: Rename old Makefiles and clarify READMEs

The old Makefiles which can be used to build this samples
directly as Linux apps and the description in the READMEs
could be quite missleading.

Building this apps in this way has nothing to do with the
Zephyr native_posix/sim targets. This is about building
them as actual Linux applications without Zephyr or any
HW models.

So let's rename the Makefiles and change the description
in the docs to avoid confusion.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>