net: sockets: add support for SO_REUSEPORT

This commits adds support for the SO_REUSEPORT socket option.

The implementation follows the behavior of BSD and tries to also follow
the specific additional features of linux with the following
limitations:
* SO_REUSEADDR and SO_REUSEPORT are not "the same" for client sockets,
  as we do not have a trivial way so identify a socket as "client"
  during binding. To get the Linux behavior, one has to use SO_REUSEPORT
  with Zephyr
* No prevention of "port hijacking"
* No support for the load balancing stuff for incoming
  packets/connections

There is also a new Kconfig option to control this feature, which is
enabled by default if TCP or UDP is enabled.

Signed-off-by: Tobias Frauenschläger <t.frauenschlaeger@me.com>