DHCP option parsing fixes

• Fix DHCP variable length option parsing for MTU responses, which
  in turns improves robustness on Amazon EC2 (fix from @yomimono
  via #48)

Bug fixes and adapt API to Mirage 1.1.1

• Catch and ignore top-level socket exceptions (#219).
• Set SO_REUSEADDR on listening sockets for Unix (#218).
• Adapt the Stack interfaces to the v1.1.1 mirage-types interface
  (see mirage/mirage#226 for details).

Rewritten interfaces that are now functorized over V1_LWT

• Rewrite of the library as a set of functors that parameterize the
  stack across the V1_LWT module types from Mirage 1.1.x. This removes
  the need to compile separate Xen and Unix versions of the stack.

Mirage 1.0 compatible version

This removes the socket interface temporarily in favour of a simple repository layout.

Switch to external Ipaddr library

• Use the Ipaddr external library and remove the homebrew
  equivalents in Nettypes.

Network manager interface cleanups

• Changes in module Manager: Removed some functions from the `.mli
  (plug/unplug) and added some modifications in the way the Manager
  interacts with the underlying module Netif. The Netif.create function
  does not take a callback anymore.

Improve TCP state machine

• Improve TCP state machine for connection teardown.
• Limit fragment number to 8, and coalesce buffers if it goes higher.
• Adapt to mirage-platform-0.9.2 API changes.