[FIXED JENKINS-21999] If a slave node does not exist it will throw a
null pointer exception. Instead we create a dummy launcher and return
that if it is unable to get the slave node.
Throw IOException if trying to launch a DummyLauncher. Restrict
DummyLauncher class to no external use.
the trunk is toward 1.559-SNAPSHOT
[FIXED JENKINS-18364] Functions.getRelativeLinkTo is called many times during rendering, so skip checking View.getItems which is expensive.
Merge branch 'rc'
[JENKINS-19055]

Adding extension point so that the master can install code that restarts JNLP slaves.
Copied from systemInfo messages
System information of slaves are now  extension points.

Existing implementations are retrofitted to the new extension point.
error handling improvement and bug fix
resurrect the original ordering
report the output in the same place the rest of the log goes
added temporary diagnostic UI
run this asynchronously so that slaves can be used right away
needs a new command that escapes Windows SCM recursive process killing
better be void
expand the scope of error handling
explaining where winsw comes from
Use the latest
moved to a separate function to reduce the serialization footprint
Integrated a new version of the module that auto-updates jenkins-slave.exe