The original motivation for the fix (to prevent users from logging in
once he's removed from the backend identity database) is legitimate,
but it affected too many users.

So as an escape hatch / non-promoted feature switch, I'm adding this
option to bring back the old behaviour.

(cherry picked from commit cf2b02dc)

Conflicts:
	changelog.html

(cherry picked from commit 87ab95e7)

[FIXED JENKINS-26601] Partly reverting #1453 to avoid hitting https://github.com/google/guice/issues/899.

(cherry picked from commit 00c858d4 into 1.599 rc)

In this environment, Util.createSymlink and .isSymlink are implemented, but resolveSymlink is not.
(cherry picked from commit 388c4b5e)

Conflicts:
	changelog.html

In this environment, Util.createSymlink and .isSymlink are implemented, but resolveSymlink is not.

[FIXED JENKINS-26601] Partly reverting #1453 to avoid hitting https://github.com/google/guice/issues/899.

Fixed JENKINS-26312, JENKINS-26298 and JENKINS-26306