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

[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.

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Commit messages:
 - Disable non-local file URLs by default

Changes: https://git.openjdk.org/jdk/pull/24657/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24657&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8353440
  Stats: 89 lines in 5 files changed: 87 ins; 0 del; 2 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