CTDB: Fix CID 1539212, update stale documentation

ctdb-tools: Fix CID 1539212 - signed/unsigned issue

>>>     CID 1539212:  Control flow issues  (NO_EFFECT)
>>>     This greater-than-or-equal-to-zero comparison of an unsigned value is always true. "p >= 0UL".
216     		while (p >= 0 && output[p] == '\n') {

This is a real problem in the unlikely event that the output contains only newlines.

Fix the issue by using a pointer and add a test to cover this case.

Update the documentation, which wasn't updated with the original change.

Fixes https://bugzilla.samba.org/show_bug.cgi?id=15438

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