[JENKINS-13202] Amelioration of problem: if the platform cannot read symlinks, fall back to visiting them as plain files or directories.

[FIXED JENKINS-15042] “Took…on master” shown for a build which ran on a slave which was since deleted.

Users are now able to update node offline messages without
bringing the node back online and offline.

Groovy should use uberClassLoader for resolving stuff.

create a Interface that Actions can implement to indicat that they
should be passed from
the matrix parent to the indiviual configurations.
This means that the SVN Parameters can be passed to them.
Partial fix for [JENKINS-14619] also needs changes in SVN SCM plugin to
fix completely.

Developed by cjo9900

This value changes every time Jenkins starts, which allows HTTP clients to reliably wait for Jenkins restart.