Cleaned up and rebased "libdivecomputer-NG" branch

[Davide DB]

As final user  I can do no other than express my utmost appreciation for
you guys trying to untangle this mess fo BT Petrel stack, OS version and
Libdivecomputer.
Without going deeper into Subsurface download it's clear to me that the
problem starts with the Android BT pairing already.

With my Petrel 2 dual stack controller

LG Xpower 220 & Android 6.0.1:
- I see the device but as I try to pair, Petrel shows a "BT INIT FAIL" and
quits.
- The phone believes that pairing was ok and shows me something like
00:13:43:5B:8F:BE address.
- Subsurface see this address but of course it doesn't work.

Nexus 5x & Android 8.1.0
- I see the device and I can pair successfully.
- Subsurface sees two addresses. Something like:
      00:13:43:5B:8F:BE
      LE:00:13:43:5B:8F:BE
- LE download doesn't start at all saying there are no new dives on device
(but the Petrel shows no sign of packet exchange)
- normal BT download works but after few dives Petrel shows a "Error
sending packet" error. My workaround is stopping the download after 2 or 3
dives so I can save them.

Nexus 7 (1st gen.) & Android 4.4
- I see the device and I can pair successfully.
- Subsurface sees one address. Something like 00:13:43:5B:8F:BE address
- normal BT download works but after few dives Petrel shows a "Error
sending packet" error. My workaround is stopping the download after 2 or 3
dives so I can save them.

About Libdivecomputer

I'm convinced that the underlying Libdivecomputer Shearwater protocol impl.
has some problem already.
In the past I downloaded raw Petrel controller data with dctool on a number
of different devices and OS and I was never able to complete a download.
At some point I always get an "Error sending packet" on Petrel. On the
contrary the Shearwater desktop/mobile download is always 100% reliable.