socket: evaluate buffer size in recv / recvfrom

When size parameter is not passed to socket:recv() or socket:recvfrom()
it will call a) or b) on the socket to evaluate size of the buffer to
store the receiving datagram. Before this commit a datagram will be
truncated to 512 bytes in the case.

a) Linux: recv(fd, NULL, 0 , MSG_TRUNC | MSG_PEEK)
b) Mac OS: getsockopt(fd, SOL_SOCKET, SO_NREAD, &val, &len)

It is recommended to set 'size' parameter (size of the input buffer)
explicitly based on known message format and known network conditions
(say, set it less then MTU to prevent IP fragmentation, which can be
inefficient) or pass it from a configuration option of a library / an
application. The reason is that explicit buffer size provided allows to
avoid extra syscall to evaluate necessary buffer size.

When 'size' parameter is set explicitly for recv / recvfrom on a UDP
socket and the next datagram length is larger then the size, the
returned message will be truncated to the size provided and the rest of
the datagram will be discarded. Of course, the tail will not be
discarded in case of a TCP socket and will be available to read by the
next recv / recvfrom call.

Fixes #3619.