[FIXED JENKINS-16215] Avoiding unnecessary downloads in FilePath.installIfNecessaryFrom

Use 'duck-typing' approach for detecting test mojos

A user running 1.467 reported:
java.lang.NullPointerException
    at hudson.model.RunMap.load(RunMap.java:216)

[FIXED JENKINS-16397] Just displaying /asynchPeople constructed the expensive People object merely to decide whether or not to display the “REST API” link.

at least interning some heavily redundant strings."

This reverts commit 08efdf5d.

Conflicts:

	changelog.html

[JENKINS-16341] INSANE testing shows that a RenderOnDemandClosure binding adds about 380k, with or wothout 08efdf5d.

Make sure that change log annotation cannot result into exception

This change does not necessarily improve total performance, since the avatar is still computed.
But (1) the computation is correctly done in the work thread, not in the HTTP response thread;
(2) the computation is done just once for a given User, which could reduce load when many AJAX checks are done.

Fixing typos on French translation (in the "Administrer Jenkins" page)

- "systême" --> "système" (5 times)
- double quotes

Conflicts:
	changelog.html

Javadoc was quite confusing as the build is actually a FAILURE if tearDown returns false and not ABORTED