socket: introduce SO_RERROR to detect receive buffer overflow

socket: introduce SO_RERROR to detect receive buffer overflow

kernel receive buffers are initially of a limited size and
generally the network protocols that use them don't care
if a packet gets lost.

However some users do care about lost messages even if not
baked into the protocol - such as consumers of route(4) to
track state.

POSIX states that read(2) can return an error of ENOBUFS so
return this error code when an overflow is detected.
Guard this with socket option SO_RERROR so that existing
applications which do not care can carry on not caring by
default.

Taken-from: NetBSD
Reviewed-by: sephe