std::net_tcp: handle ipv6 addresses

[graydon]

As per fixme, not presently supported.

[olsonjeffery]

this one is mine.. im working on it in olsonjeffery/rust tcp_cleanup

only noticed because of the merge conflict :)

trying to land a pull request in the next few days. sorry for the silence.

[graydon]

no worries. thanks for the ongoing effort!

[bblum]

libextra::timer and libextra::net_tcp are deprecated pending #6435.

[metajack]

visiting for triage. can we close this since the old scheduler is gone?

[anasazi]

Yes, IPv6 support for TCP and UDP was part of the new runtime.