Remove unnecessary executable bits on source files

Fix windows-slaves-plugin

2669e4a8 extracted windows-slaves code
to a separate plugin.
However in the process, it introduced a dependency conflict.
core -> j-interop -> jcifs (1.2.19)
whereas
windows-slaves-plugin -> ... -> jcifs (1.3.17-kohsuke-1)

This was causing NoSuchMethodError while launching windows slaves

Introduce `OfflineCause.UserCause`

Let -Dignore.random.failures work on UpdateCenterTest, which is currently failing and so blocking validated merge.

ComputerLauncher.checkJavaVersion should be ensuring that the slave is Java 6+, since Java 5 is no longer supported for a slave agent.

(What is ${baseUrl}? ${rootURL} was meant.)

[FIXED JENKINS-21448] Report number of jobs in any container