Fixed race waiting for ARP response

We didn't start watching the condition until after sending the packet.
By then, it may have already sent the notification.
Use Lwt thread for ARP response

Using a Lwt_condition here is unnecessary complicated, and risks races
such as the one fixed in the previous commit.
Dhcpv4: Decouple from Ipv4

The code to set the ip address, netmask, and gateways from the
received offer moved to the direct Stackv4.
Stackv4: keep listening for DHCP offers
Adding tracing labels and counters
bump working version to 2.0.3
Merge branch 'dhcp-offers' of git://github.com/nojb/mirage-tcpip
Sync CHANGES
In the DHCP configuration thread, do not block indefinitely.

Terminate after the first offer is received, which brings us back
to the same functionality as before 4c7b0c7b1114fb71152fd8a95c
was applied.  Without this patch, TCP services never get a chance
to start because the configuration never ends.
Build Xen stubs separately, with CFLAGS from mirage-xen
Merge branch 'tracing2' of git://github.com/talex5/mirage-tcpip into talex5-tracing2

Conflicts:
	lib/META
	lib/arpv4.ml
	setup.ml
Merge branch 'talex5-tracing2'

Conflicts:
	_oasis
	_tags
	lib/META
	setup.ml
bump working version to 2.1.0
Add tracing labels for some mvars
Merge pull request #89 from talex5/mvars

Add tracing labels for some mvars
Merge branch 'master' of https://github.com/mirage/mirage-tcpip into merge

Conflicts:
	_oasis
	_tags
	dhcp/dhcp_clientv4.ml
	lib/META
	lib/arpv4.ml
	lib/ethif.ml
	opam
	tcp/wire.ml