Start synchronising the xen libs with the userspace xenctrl ones

Rename Evtchn -> Eventchn
 -- Evtchn is the better name but Eventchn has been in xen-unstable for
    a while now
Remove xen Blkif from the build: will switch to ocaml-xen-block-driver
 -- perhaps we should put both xen and unix blkif in 'ocaml-block-driver'
    to make the package dependencies simpler?
Grant table 'map', 'mapv' (as would be used by a backend)

Unfortunately it's not possible to synchronise the xenctrl 'grant share'
(ie exporting of grant references as-in blkfront) because the mirage
implementation allows the sharing of an existing buffer but the xenctrl
version does not.
Remove the xen blkif implementation: this now resides in ocaml-xen-block-driver
Allow device providers to be added later

Previously a master device_t took a snapshot of the provider list at
a single point in time, preventing providers being registered later.

Instead we allow providers to be registered whenever is convenient,
and start per-provider threads on demand.
netif: update to new shared ring signature
Add a simple 'resume hook' mechanism for xen devices

In-tree Netif adds a resume hook.
xen: installing mirage provides a virtual package 'xenctrl'

An xen device driver can now depend on 'xenctrl' and build both
in mirage and in Linux userspace.
xen: installing mirage provides a virtual package 'xenctrl'

An xen device driver can now depend on 'xenctrl' and build both
in mirage and in Linux userspace.
Merge pull request #27 from djs55/sync-eventchn-with-xen

Move to out-of-tree blkfront
Merge pull request #29 from djs55/xenctrl

xen: installing mirage provides a virtual package 'xenctrl'
Add missing file
Merge pull request #32 from djs55/sync-eventchn-with-xen

Add xenctrl package, properly this time
Only the xen backend needs the META.xenctrl file
Merge pull request #33 from djs55/sync-eventchn-with-xen

Only the xen backend needs the META.xenctrl file
Unix: add dependency on tuntap in ocamlfind META
Merge pull request #34 from djs55/sync-eventchn-with-xen

Unix: add dependency on tuntap in ocamlfind META
Replace linux pcap stubs accidentally lost in beefa72e97b1b55e0d2e4b214e8c411b65e097a2

The tuntap stubs have moved to the tuntap package.
Merge pull request #35 from djs55/linux-pcap-stubs

Replace linux pcap stubs accidentally lost in beefa72e97b1b55e0d2e4b214e...
Add a 'make uninstall' target

Since we install a 'xenctrl' package in the xen backend but not the unix
one, we need to have a backend-specific uninstall to clean everything up.
Merge pull request #36 from djs55/uninstall

Add a 'make uninstall' target
Remove internal xen activations functions from the interface