Code review request: Drop obsolete RFC-791 markings for QoS markings

[Philip Prindeville]

Here's the bug and proposed patch.  It's pretty trivial.

https://bugzilla.mindrot.org/show_bug.cgi?id=1856


Quoting RFC-2474:

    A replacement header field, called the DS field, is defined, which is intended to supersede the existing definitions of the IPv4 TOS octet [RFC791] and the IPv6 Traffic Class octet [IPv6]. [...] The structure of the DS field shown above is incompatible with the existing definition of the IPv4 TOS octet in [RFC791].

and:

    No attempt is made to maintain backwards compatibility with the "DTR" or TOS bits of the IPv4 TOS octet, as defined in [RFC791].

Also note that the patch that was originally proposed (but not accepted as-is) for bug 1733 attempted to disallow user setting of the QoS bits, also in accordance with RFC-2474:

    Correct operational procedure SHOULD follow [RFC791], which states: "If the actual use of these precedence designations is of concern to a particular network, it is the responsibility of that network to control the access to, and use of, those precedence designations."