Add CTDB tunables.d/ directory

As a vendor, I really want a way of keeping my required tunables settings separate from those a customer might set. I first decided to add ctdb.tunables.local. That seemed inflexible and a colleague suggested adding includes. Although easy on the surface (just treat the include key/tunable as special, like smb.conf does), you need to detect loops, have a maximum include depth (I had decided on 1), decide whether to support wildcards and even consider possible security implications. I also noticed that other software (e.g. sysctl) seems to have moved on from includes to using (sometimes multiple) *.d/ directories.

So, tunables.d/*.tunables it is!

Some of the changes improve logging for the multi-file situation. Unit testcases have been added to cover the new uses. I have hand tested combinations of working/broken tunables files (including missing tunables.d/ directory) with local daemons. Both types of testing have been run with vagrant -q valgrind -q.

Checklist

• Commits have Signed-off-by: with name/author being identical to the commit author
• (optional) This MR is just one part towards a larger feature.
• (optional, if backport required) Bugzilla bug filed and BUG: tag added
• Test suite updated with functionality tests
• Test suite updated with negative tests
• Documentation updated
• CI timeout is 3h or higher (see Settings/CICD/General pipelines/ Timeout)

Reviewer's checklist:

• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to README.Coding.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code