Differentiated Services support in SSH

I've noticed that OpenSSH uses the now-obsolete original definitions of
the IP TOS (Type of Service) field. In packet.c, IPTOS_LOWDELAY is
chosen if the connection is interactive, IP_THROUGHPUT otherwise.

Several years ago the IETF completely redefined this field to support a
new "differentiated services" architecture. The low two bits are
reserved for explicit congestion notification (ECN), which conflicts
with some of the previous IPTOS definitions (namely IPTOS_MINCOST, which
fortunately OpenSSH does not use). The upper 6 bits are available for a
Differentiated Services Code Point (DSCP) value that indicates to the
routers how this packet is to be handled.

Because these DSCP values are not yet well defined, and they're allowed
to have local significance, I would like to add config options to set
the DSCP values that SSH should use on interactive and non-interactive
connections. I can't find any mention of this on this list or in the bug
reports, but I wanted to avoid any duplication of effort in case someone
else is already working on this.

If no one is, then I'll implement it and submit the patches.

--Phil
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