RFR: 8353440: Disable FTP fallback for non-local file URLs by default [v3]

[Eirik Bjørsnøs]

> Please help review this PR which disables the unspecified but long-standing feature where an `FtpURLConnection` is opened as a fallback for non-local file URLs.
> 
> Before this change, if a file URL has a non-local host component, say `file://remotehost/folder/data.txt`, then the  implementation would attempt opening an FTP connection to `remotehost`. After this change, such URLs will be rejected with a `MalformedURLException`, unless the FTP fallback feature is explicitly re-enabled via a system property.
> 
> This change was initially discussed here: https://mail.openjdk.org/pipermail/net-dev/2025-March/025988.html
> 
> See the above discussion and CSR draft JDK-8354678 for the motivation for this change. I plan to update the CSR pending an initial round of review of this PR.
> 
> This PR:
> 
> * Changes file URL `Handler::openConnection` implementation for unix/windows to throw `MalformedURLException`, unless the FTP fallback feature is explicitly enabled by configuration.
> * Introduces a new system property `sun.net.www.protocol.file.ftp-enabled` which when set to `true` re-enables the feature.
> * Updates the existing test `NonLocalFtpFallback` to enable the feature via said system property.
> * Adds a new test `NonLocalFtpFallbackDisabled` verifying that a `MalformedURLException` is thrown by default for a non-local URL host component.
> 
> I have added a Release Note as a subtask in the JBS issue, this also needs a review.

Eirik Bjørsnøs has updated the pull request incrementally with one additional commit since the last revision:

  Document jdk.net.file.ftpfallback as a Networking Property

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Changes:
  - all: https://git.openjdk.org/jdk/pull/24657/files
  - new: https://git.openjdk.org/jdk/pull/24657/files/8be0010f..515f4789

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=24657&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=24657&range=01-02

  Stats: 10 lines in 1 file changed: 10 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/24657.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24657/head:pull/24657

PR: https://git.openjdk.org/jdk/pull/24657