Set default route MTU when using multihop on Linux

[dlon]

This prevents fragmentation when multihop is enabled.

Fix DES-483.

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This change is 

[linear]

DES-483 Specify lower MTU on default path for multihop

[pinkisemils]

Reviewed 5 of 6 files at r1.
Reviewable status: 5 of 6 files reviewed, 1 unresolved discussion (waiting on @dlon)

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talpid-wireguard/src/lib.rs line 484 at r1 (raw file):

        if config.peers.len() > 1 {
            if config.peers.len() != 2 {

I presume we would support an arbitrary amount of peers now?

[dlon]

Reviewable status: 4 of 9 files reviewed, 1 unresolved discussion (waiting on @pinkisemils)

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talpid-wireguard/src/lib.rs line 484 at r1 (raw file):

Previously, pinkisemils (Emīls Piņķis) wrote…

I presume we would support an arbitrary amount of peers now?

I've made Config less generic to remove all of the hidden assumptions.

[pinkisemils]

Reviewed 1 of 5 files at r3, all commit messages.
Reviewable status: 5 of 9 files reviewed, 1 unresolved discussion (waiting on @dlon)

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talpid-wireguard/src/lib.rs line 920 at r4 (raw file):

            let ip_overhead = match route.prefix.is_ipv4() {
                true => IPV4_HEADER_SIZE,

Do we adjust the MTU size when connecting over IPv6 instead of IPv4 too?

[pinkisemils]

Reviewed 3 of 5 files at r3, 1 of 1 files at r4.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @dlon)