@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.)