[FYI] Experimenting replacing cwrap with namespaces in selftest

[Tim Beale]

Hi,

Just FYI, something we've been experimenting with in the background is
replacing some of the cwrap usage in selftest. We've tried removing
socket-wrapper/resolv-wrapper and using containerization via linux
namespaces instead.

Namespaces mean that each testenv can have its own /etc/resolv.conf and
have its own unique hostname, etc.

There's more details about it here:
https://wiki.samba.org/index.php/User/Timbeale/Replace_cwrap_with_namespaces

Note there are still some major limitations involved in using namespaces
for things like gitlab CI or autobuild. Currently all I've really got is
a proof of concept that namespaces can work for testenvs. (The credit
for the prototyping really goes to Garming).

Over the next few weeks I plan to tidy up the changes I've got. I think
they can live alongside cwrap in selftest without degrading/complicating
the code too much. In the short-term, namespaces would probably be handy
for things like the customdc testenv, or manual developer testing (i.e.
you could potentially point other tools at your testenv DC).

Cheers,
Tim