Dav: improved path validation for COPY and MOVE operations

[saikrishnakumarreddy]

Improved path validation for COPY and MOVE operations

The COPY and MOVE handler did not validate whether source and
destination paths referred to the same resource or a parent-child
collection relationship, which could corrupt or destroy files.

Now 403 is returned if paths match or one is a prefix of the other.

Testing

Describe how you tested the change (manual testing, automated tests,
regression tests, etc.).

• Manual Testing
• Functional Testing (nginx-tests)

Checklist

Before submitting this PR, please confirm:

• I have read the CONTRIBUTING guidelines
• All existing tests pass
• My branch is rebased on the latest master
• This PR targets the master branch from my fork
• My commit message follows NGINX standards (imperative mood, clear
  subject, references related issue if applicable, and contains only
  relevant changes)

[pluknet]

The commit message could be less verbose for such a change, but it's good this way too.
Some nitpicking on it.

Dav: added path validation for COPY and MOVE operations

improved?

Because certain validation was already there.

Fix: after resolving ...

Such wording is not used, I suggest to change it to something like:

Now, after resolving ...

or

The fix is, after resolving ...

Otherwise, looks good.

[pluknet]

The checks don't catch a path name with multiple consecutive slashes, which maps to the same file.
What we can do here is to merge slashes then compare, or write such a compare function that would take this into account.