Shrink dynamic port range

[dadgar]

This PR shrinks the dynamic port range to not overlap with most
operating systems ephemeral port range: https://en.wikipedia.org/wiki/Ephemeral_port

Fixes #2402

[hynek]

Since I just ran into #2402 in prod multiple times with nomad 0.5.6, is there any chance you back port this to 0.5.x or does everyone have to upgrade to 0.6.x?

[dadgar]

@hynek You could checkout the v0.5.6 tag and merge this change and build a custom binary if you don't want to upgrade to 0.6.0

[hynek]

Heh I do want but I do upgrade but you know how it is; major updates take time.

I was just wondering whether there'll ever be a 0.5.x bug fix release but I suppose that's a no. :)

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