net: sockets: Add configurable option to provide raw POSIX API names

With CONFIG_NET_SOCKETS_POSIX_NAMES=y, "raw" POSIX names like
socket(), recv(), close() will be exposed (using macro defines).
The close() is the biggest culprit here, because in POSIX it
applies to any file descriptor, but in this implementation -
only to sockets.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>