Support SO_MAX_PACING_RATE.

Support SO_MAX_PACING_RATE.

SO_MAX_PACING_RATE is the newfangled socket option from Eric Dumazet,
used with the new fq packet scheduler in Linux. It allows you to set
a max rate for the socket (presumably a stricter upper bound than the
RTT-based estimate from the kernel), delivering pacing without having
to resort to the relatively complex mark setup. It seems to enter
the Linux kernel in 3.13 at the earliest; not unlikely even later.

In time, fwmark will be deprecated, but the implementation of TCP pacing in
Linux is still a bit shaky (especially with not-always-filling applications
like streaming), so fwmark will stay the primary solution for now.