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In addition to SO_REUSEADDR, that we were setting before, via
AllowAddressReuse(), for sockets for use with multicast, now also set
SO_REUSEPORT whenever that option is defined. On some non-Windows
platforms, this is needed for multicast messages to be received by all
sockets listening for them. This option is only set in a new
AllowAddressSharingForMulticast() method as the SO_REUSEADDR and
AllowAddressReuse() have other non-multicast uses where SO_REUSEPORT
would not be appropriate.

Also, for non-Windows platforms, bind the multicast sockets to the
multicast group address rather than to a wildcard. This will make us
more likely to only get messages from the relevant multicast group as
some platforms will otherwise give the socket any multicast messages
from groups joined by any sockets in the system. No change on Windows
where binding to a multicast group is not allowed.

TBR=thestig@chromium.org

Change-Id: I24d9d4a0d85134074b2deb14914a41109d7f724a
Bug: 899310
Reviewed-on: https://chromium-review.googlesource.com/c/1298298
Commit-Queue: Eric Orth <ericorth@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603180}