RFR: 8324635: (zipfs) Regression in Files.setPosixFilePermissions called on existing MSDOS entries

[Eirik Bjørsnøs]

Please review this PR to fix to a regression in ZipFileSystem, introduced by JDK-8322565 in PR #17170.

When `Files.setPosixFilePermissions` is called on an existing MSDOS entry, then `Entry.posixPerms` field will be -1 (all 1s in binary). The logic introduced by JDK-8322565 did not account for this and incorrectly sets the leading seven bits to all ones instead of all zeros.

The fix is to introduce a branch for `Entry.posixPerms` being -1 and deal with that case separately.

The PR adds a test case to `TestPosix` which reproduces the regression. While visiting this test, I also fixed an incorrect spelling of `setPosixFilePermissions` (also introduced by #17170).

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Commit messages:
 - If the entry currently has no permissions set, don't try to merge permission bits with existing -1 bits

Changes: https://git.openjdk.org/jdk/pull/17556/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17556&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8324635
  Stats: 31 lines in 2 files changed: 29 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/17556.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17556/head:pull/17556

PR: https://git.openjdk.org/jdk/pull/17556