net: sockets: Use only Zephyr POSIX headers

Use only Zephyr specific POSIX header files so that the whole
system is getting values from the same files. There was an issue with
native_sim run of tests/net/socket/af_packet which had O_BLOCKING set
to 0x4000 from include/zephyr/posix/fcntl.h, but then the file
subsys/net/lib/socket/sockets.c was having O_BLOCKING set to 0x0800
because different header files were used.

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