RFR: 8322565 (zipfs) Files.setPosixPermissions should preserve 'external file attributes' bits [v3]

[Eirik Bjørsnøs]

> This PR suggests that `Files.setPosixPermissions`as implemented by `ZipFileSystem` should preserve the leading seven bits of the 'external file attributes' field. These bits contain the 'file type', 'setuid', 'setgid', and 'sticky' bits. These are unrelated to permissions and should not be modified by this operation.
> 
> The fix is to update `Entry.readCEN` to read all 16 bits instead of just the trailing 12 and to update `ZipFileSystem.setPermissions` to preserve the leading 7 bits when updating the trailing 9 permission-related bits of the `Entry.posixPerms` field.
> 
> The PR adds a new test `TestPosix.preserveRemainingBits()` which verifies that the leading 7 bits are not affected by `Files.setPosixPermissions`. This test also verifies that operations not related to POSIX, such as Files.setLastModifiedTime does not affect the 'external file attributes' value.
> 
> Note that this PR does not aim to preserve the leading seven bits for the case when `Files.setPosixPermissions` is called with a `null` permission set. (The implementation currently interprets this as a signal that the 'external file attributes'  should not be populated and the 'version made by' OS will be MSDOS instead of Unix)

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

  Extract testing of the Files.setPosixFilePermissions operation and the Files.setLastModifiedTime operation into separate test methods.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/17170/files
  - new: https://git.openjdk.org/jdk/pull/17170/files/524159f3..e4a505fa

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17170&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17170&range=01-02

  Stats: 53 lines in 1 file changed: 35 ins; 11 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/17170.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17170/head:pull/17170

PR: https://git.openjdk.org/jdk/pull/17170