net: sockets: Add options to control DSCP/ECN value

Add new socket options IP_TOS and IPV6_TCLASS which allows to set
DSCP/ECN values on a socket for an outgoing packet IPv4/IPv6 headers.

The options are compatible with Linux behaviour, where both DSCP and ECN
are set with a single socket option.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>