s3-libsmbclient Add comments to describe the behaviour of DEBUG()

s3-libsmbclient Add comments to describe the behaviour of DEBUG()

This isn't quite what you would expect from this interface, but actually
avoids some really nasty situations if you ever have more than one
libsmbclient context in a process.

In the real world, if you have asked for DEBUG() to stderr in one part
of the code, you will want it globally, even in a different thread
(which in the past would have rest everything to stdout again, at
least while starting up).

Andrew Bartlett