net: lib: sockets: Switch to use fdtable

Previously the "socket file descriptors" were just net_context
pointers cast to int. For full POSIX compatibility and support
of generic operations line read/write/close/fcntl/ioctl, the
real file descriptors should be supported, as implemented by
fdtable mini-subsys.

Socket implementation already has userspace vs flatspace dichotomy,
and adding to that ptr-fds vs real-fds dichotomy (4 possible cases)
is just too cumbersome. So, switch sockets to real fd's regardless
if full POSIX subsystem is enabled or not.

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