[PATCH 0 of 7 v5] Introduce the tapback daemon (most of blkback in user-space)

[Thanos Makatos <thanos.makatos@citrix.com>]

This patch series introduces the tapback daemon, the user space daemon that
acts as a device's back-end, essentially most of blkback in user space. The
daemon is responsible for coordinating the front-end and tapdisk. It creates
tapdisk process as needed, instructs them to connect to/disconnect from the
shared ring, and manages the state of the back-end.

The shared ring between the front-end and the tapdisk is provided by a piece of
code that lives inside the tapdisk and will be introduced by the next patch
series.
 
Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>

---
Changed since v1:
The series has been largely reorganised:
* Renamed the daemon from xenio to tapback.
* Improved description in patch 0.
* Merged structures and functions.
* Disaggregated functionality from the core daemon source file to smaller ones
  in order to facilitate the review process and improve maintenance.

Changed since v2:
  * Added a new patch that ignores tapback binaries.
  * For the rest of the patches, see the description in each patch.

Changed since v3:
  * Replace the minor number with type:/path/to/file where necessary.
  * Create the daemon's control socket.