net: sockets: Deprecate CONFIG_NET_SOCKETS_POSIX_NAMES

The CONFIG_NET_SOCKETS_POSIX_NAMES option is marked as deprecated in
favor of using normal POSIX socket API includes found under the
include/zephyr/posix directory. If you want to use BSD socket API calls,
you need to select POSIX_API and use the socket headers found in the
POSIX subsystem. If you do not want to or cannot enable POSIX_API,
then you must use zsock_ prefix when working with BSD socket calls.

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