Redox unix

See merge request !35

fix return statement of else block in dup function

See merge request !34

Update Cargo.lock

See merge request !32

A 'precise' cargo update of tokio is needed for the build to succeed.

cargo update

See merge request !31

Fixed packet buffer logic error in smoltcp

Update smoltcp

Closes #30

See merge request redox-os/netstack!30

* Use new SchemeBlock trait

* Use EAGAIN over EWOULDBLOCK

* icmp.rs add udp branches to read and write

* add write_buf implementation to icmp udp port unreachable

* add udp port support