Unify/Modernize TOS to DSCP standards

[Artoria2e5]

The set of pre-named TOS values are now renamed to the latest DSCP
standards, with traffic classes and everything exciting. To keep
everything in the same place, a segment has been added to net.h to keep
the currently named values.

A result of these changes is that "lowcost" is probably no longer
harmless, as it now encourages the router to preferentially drop the
packages when bandwidth requires so. "lowdelay" is assigned to a pretty
high AF class for SIP and stuff. I am not sure at all about the
"throughput" assignment. I mean, the whole point of DSCP-fication is
translating from the old ToS bits to a more precedence-based notation,
and precedence is supposed to lie alongside the old 4 ToS bits,
not be part of them...

A funny interaction between the AFxy and the old ToS fields lies in
the old "reliability" (0x08) field. All odd numbers of y (1, 3 : low,
high) switches it on. If you spend 5 more minutes on it you can probably
come up with "pun" values that hold similar meanings in DSCP and
Prec/ToS.

By removing the IPv6 override, I should have kind of satisfied the #692
request.

[Artoria2e5]

A modest request: Transmission can probably remove the non-Win32 check for IP_TOS. Win32 does bail out on TOS settings if a registry key is not set, but it calls it a success anyway. Since the commit uses DSCP completely, we can completely satisfy whatever Windows is trying to have people move to since Win2k. Oh forget about it there's a lot of GQoS/QoS2 black magic after Windows 5.x. This post does suggest that the registry key helps, but it's just too much. Add a line in the documentation that says there's no ToS for Win32.

Setting IPV6_TCLASS is still known to be bad though. Let's keep that check, at least until some cutting-edge Windows 10 folk has seen it working.

[neomerx]

Can you please check if I set a raw number instead of "lowcost"/"mincost"/etc it will be used?

[Artoria2e5]

The strtol part should do just that.

[Artoria2e5]

Oh damn it there's a thing called format_tos

[ckerr]

Nice work. I have a couple of implementation change requests, but the core idea LGTM.

[ckerr]

It looks like the previous code was trying to set reasonable defaults for lowcost, throughput, etc, but I don't understand why it doesn't have a default: dscp = tos; to let users specify their own custom values. 👍 on the new code passing this through directly.