Also fix :send() and :write()

Do not terminate the server on accept error.

This patch also fixes EINTR and EWOULDBLOCK handling for
socket:read() and socket:write() wrappers.

open(, O_CREAT | O_DIRECT, ) creates a file and then fails with
EINVAL in case if the underlying filesystem doesn't support O_DIRECT.
This behaviour is stupid and unexpected.

Provide fine-grained keepalive options on Linux, which
supports per-socket defaults.

Fix a bug with trying to set keepalive options for UDP
sockets - don't try to do it.