Avoids rechecking at every startup, mainly relevant during interactive testing when restarts may be quite frequent.

In this case we are probably interested in looking at the output as it arrives in real time.
Can always be overridden on the command line if desired.
(cherry picked from commit 44a8ec11)

Also moving this switch to security settings.

(cherry picked from commit 6318b8d8)

(cherry picked from commit e4c5c710)

(cherry picked from commit df9c92e4)

(cherry picked from commit 1c99cc1a)

loadJSON does not support parsing HTML.
Nor should it need to, since UC URLs are supposed to be to the JSONP file.
The .html extension is added automatically by downloadService when postBack is supported.
(cherry picked from commit d6b37a94)

Conflicts:
	test/src/test/java/hudson/model/UpdateSiteTest.java
	test/src/test/resources/hudson/model/update-center.json.html

(cherry picked from commit fb524145)

@kohsuke prefers that the option to disable signature checks not even be presented unless -Dhudson.model.DownloadService.noSignatureCheck is used.
(cherry picked from commit 54c85007)

Be a bit paranoid and ensure the admin has CONFIGURE_UPDATECENTER before turning off signature checks.

(cherry picked from commit 1262d882)

(cherry picked from commit 1ac77750)

Conflicts:
	core/src/main/java/hudson/model/DownloadService.java
	core/src/main/java/hudson/model/UpdateSite.java

Ugly hack to fix destroyProcess for Java8

(cherry picked from commit 19640e7b)

Integrated the new version of XStream that contains the fix.

(cherry picked from commit 585eb87c)

Conflicts:
	changelog.html

This is already handled by ListView.Listener (other kinds of views are now responsible for their own listeners).
And handled better—if there are no actual changes
(because you have no such views, or they do not mention this job), there is no need to save.
Also moving the actual saving in those remaining cases out of the lock on the ListView itself, just in case.

[JENKINS-21984] Disabling cert check on updateDirectlyWithJson since the certificate is now expired, and it is no good to have a test which is known to start failing at a particular time in the future!