[FIXED JENKINS-14667] JDK installation failed on some slaves with InvalidClassException: hudson.tools.JDKInstaller$Platform$1; local class incompatible: …

(cherry picked from commit 43ff1688)

From what I was told by Emanuele today in the office hours, this came up
during the Copenhagen hackathon.

Put slave back online automatically, if there's enough disk space again ...

Javadoc improvements inspired by “How to stop passwords used by my scm plugin from appearing the log?” on jenkinsci-dev.

Added callable wrapper to actAsync

make AbstractInterceptorCallableWrapper a static inner class for usability in plugins

Prefer the postMessage version to retrieve the metadata so that we can leave the existing JSON format as-is for backward compatibility with pre-1.424 Jenkins.

Updated/changed German translations

@see hudson.maven.MavenUtil#createEmbedder

extension point to intercept FileCallable.act

[FIXED JENKINS-13202] Try to use NIO.2 (Java 7) methods to work with symlinks.

Conflicts:
	changelog.html

remove empty Maven installations before saving them
Thanks!

Integrated a fix Jesse made in stapler.

(Not doing so for resolveSymlink since it would require an awkward four-way return value:
String, null (not a symlink), IOException, <something else> for Java 5/6.)