Deny sending and receiving shared media redirects

[raja-grewal]

As per #279 (comment).

Changes

Set sysctl net.ipv4.conf.*.shared_media=0

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[ArrayBolt3]

Seems fine to me, worst-case scenario it makes the network less efficient from my research, and there seems to be a good argument for setting this option.

Edit: Minor nitpick, but there seems to be some extra whitespace (two spaces in a row) in the comments above the actual sysctl option.