RFR: 8267108: Alternate Subject.getSubject and doAs APIs that do not depend on Security Manager APIs [v3]

[Weijun Wang]

> New `Subject` APIs `current()` and `callAs()` are created to be replacements of `getSubject()` and `doAs()` since the latter two methods are now deprecated for removal.
> 
> In this implementation, by default, `current()` returns the same value as `getSubject(AccessController.getCurrent())` and `callAs()` is implemented based on `doAs()`. This behavior is subject to change in the future once `SecurityManager` is removed.
> 
> User can experiment a possible future mechanism by setting the system property `jdk.security.auth.subject.useTL` to `true`, where the `callAs()` method stores the subject into a `ThreadLocal` object and the `current()` method returns it (Note: this mechanism does not work with principal-based permissions).
> 
> Inside JDK, we’ve switched from `getSubject()` to `current()` in JGSS and user can start switching to `callAs()` in their applications. Users can also switch to `current()` but please note that if you used to call `getSubject(acc)` in a `doPrivileged` call you might need to try calling `current()` in a `doPrivilegedWithCombiner` call to see if the `AccessControlContext` inside the call inherits the subject from the outer one.

Weijun Wang has updated the pull request incrementally with one additional commit since the last revision:

  address comments from Sean and Bernd

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5024/files
  - new: https://git.openjdk.java.net/jdk/pull/5024/files/2f862e02..a29c8d93

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5024&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5024&range=01-02

  Stats: 39 lines in 5 files changed: 0 ins; 11 del; 28 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5024.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5024/head:pull/5024

PR: https://git.openjdk.java.net/jdk/pull/5024