Introduce Option.SocketReuseAddrAndPort which, if set, will lead to the
SO_REUSEPORT socket option being set on the listening socket on
Unix-like OSes. This also sets SO_REUSEADDR which is already the default
in net.Listen (see net.setDefaultSockopts).
Setting these options increases the chance to re-bind() to the same
address and port upon agent restart if Options.Addr is set.
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>