posix: sockets: Implement SO_RCVTIMEO option in setsockopt

AwaLWM2M needs to be polled regularly to check for incoming data.
Since RIOT only supports timeout at the GNRC sock layer while
the network abstraction for RIOT in AwaLWM2M uses the posix layer,
this causes RIOT to be blocked waiting for data that never arrive.

This commit implements only the SO_RCVTIMEO option in setsockopt to
allow users to set a receive timeout for a socket at the posix layer.

Signed-off-by: Francois Berder <francois.berder@imgtec.com>