Add IPv6 stack

Add IPv6 support. This changeset minimises interface changes to the existing
STACKV4 interfaces to faciliate a progressive merge. The only visible
interface changes are:

• IPV4.set_ipv4_* functions have been renamed IPV4.set_ip_* because they
  are shared between IPV4 and IPV6.
• IPV4.get_ipv4 and get_ipv4_netmask now return a list of Ipaddr.V4.t
  (again because this is the common semantics with IPV6.)
• Several types that had v4 in their names (like IPV4.ipv4addr) have lost
  that particle.

Improved DHCP logging

Improve console printing for the DHCP client to output line breaks properly on Xen consoles.

Add profile tracing, and better Xen stub compilation

• Build Xen stubs separately, with CFLAGS from mirage-xen 2.1.0+.
  This allows us to use the red zone under x86_64 Unix again.
• Adding tracing labels and counters, which introduces a new dependency on the
  mirage-profile package.

Remove ARP race condition and simplify DHCP client

• Fixed race waiting for ARP response (#86).
• Move the the code that configures IPv4 address, netmask and gateways
  after receiving a successful lease out of the Dhcp_clientv4 module
  and into Stackv4 (#87)

Support IPv4 multicast addresses and stability fixes

• Add IPv4 multicast to MAC address mapping in IPv4 output processing (#81 from Luke Dunstan).
• Improve formatting of DHCP console logging, including printing out options (#83).
• Build with -mno-red-zone on x86_64 to avoid stack corruption on Xen (#80).

Improve behaviour under heavy load

• Fixed race condition in the signalling between the rx/tx threads under load.
• Experimentally switch to immediate ACKs in TCPv4 by default instead of delayed ones.

Mirage 2.0 compatible TCP/IP release, and socket backend fixes

• Moved 1s complement checksum C code here from mirage-platform.
• Depend on Console_unix and Console_xen instead of Console.
• [socket] Do not return an Eof when writing 0-length buffer (#76).
• [socket] Accept callbacks now run in async threads instead of being serialised (#75).

More robust TCP options parsing

• Quieten down the stack logging rate by not announcing IPv6 packet discards.
• Raise exception Bad_option for unparseable or invalid TCPv4 options (#57).
• Fix linking error with module Tcp_checksum by lifting it into top library (#60).
• Add opam file to permit easier local pinning, and fix Travis to use this.

Stability fixes to TCPv4 and DHCP handling

• Ensure that DHCP completes before the application is started, so that
  unikernels that establish outgoing connections can do so without a race.
  (fix from Mindy Preston in #53, followup in #55)
• Add echo, chargen and discard services into the examples/
  directory. (from Mindy Preston in #52).
• [tcp] Fully process the last ACK in a 3-way handshake for server connections.
  This ensures that a FIN is correctly transmitted upon application-initiated
  connection close. (fix from Mindy Preston in #51).

Expose IPV4 module in the STACKV4 functor

• Expose IPV4 through the STACKV4 interface.

This requires the corresponding mirage-types package in Mirage 1.1.2