(cherry picked from commit 93467c3f)

Clarify PluginManager#getPlugin() and Jenkins#getPlugin() methods

The test was not updated to reflect the URL change. This is blocking a
release.

Update the Agent => Master security link to point to a non-Apache redirect

[INFRA-1032] hack around by forcing user.name

Make compilable with JDK 9 javac

This help text points to a very old redirect which is only editable in our
infrastructure Puppet code (which will at some point in the distant future be
removed).

Changing to jenkins.io/redirect means that the destination of this redirect
is easily modified by pull requests to
https://github.com/jenkins-infra/jenkins.io

See also jenkins-infra/jenkins.io#520

[FIX JENKINS-40894] Restore unstableReturn in readResolve

Polish translations

[FIXED JENKINS-40886] - Enable the JNLP4 protocol by default

[JENKINS-40813] More strongly worded message on enabling security

[FIX JENKINS-40813] Better message on 'Enable security' option

When an empty Freestyle job config gets submitted in the default configuration of Jenkins 2.35, the data is being saved to the disk *8 times*. All of them happen in this code: https://github.com/jenkinsci/jenkins/blob/master/core/src/main/java/hudson/model/Job.java#L1227-L1246

* setDisplayName
* Project#getBuildWrappersList().rebuild (onModified handler)
* Project#getBuilderList().rebuild (onModified handler)
* Project#getPublisherList().rebuild (onModified handler)
* AbstractProject#makeDisabled
* AbstractProject#setScm
* AbstractProject#triggers.replaceBy
* final save()

There is not so much sense to save partial configurations to the disk due to the risk of data inconsistency there. This change just wraps the config submission section of the job into the BulkChange clause.
(cherry picked from commit a0262d2f)

[JENKINS-40666] - Correctly state that Jenkins will refuse to load plugins.
(cherry picked from commit d05752a0)

The re-check updatecenter button should be visible even if there are
currently no pending updates.
(cherry picked from commit 8634965a)

* [FIX JENKINS-39268] Properties are not passed to Maven command by Maven build step

* [FIX JENKINS-39268] Properties are not passed to Maven command by Maven build step

* [FIX JENKINS-39268] Properties are not passed to Maven command by Maven build step

* [FIX JENKINS-39268] Changed order in which properties are appended to command line: properties appended later win in case of conflicts

* [FIX JENKINS-39268] More precise messages in some assertions

* [FIX JENKINS-39268] Cleanup unused imports

* [FIX JENKINS-39268] Added functional tests for Maven task.

(cherry picked from commit 138ce3d8)

(cherry picked from commit 4f814c0a)

(cherry picked from commit 7c2e1b2e)

[FIX JENKINS-32358] Correctly compute plugin name when multiple sources are passed to install-plugin CLI command

(cherry picked from commit ef8ddd8a)