In this environment, Util.createSymlink and .isSymlink are implemented, but resolveSymlink is not.

Note some nonnull parameters

[JENKINS-26643] DescribableList.buildDependencyGraph should catch RuntimeException

Add extra info if the slave can not restart correctly.

If the service can not reliably restart itself add link to the wiki with
some information about how to fix it.

[FIXED JENKINS-26601] Partly reverting #1453 to avoid hitting https://github.com/google/guice/issues/899.

Trying to diagnose hung connection in UC download.

Observed in 1.580.1 on credentials-plugin (which you would think would be tiny and fast to get):
"Update center installer thread [#1]" #70 daemon prio=5 os_prio=0 tid=0x00007f5634815800 nid=0x64a8 runnable [0x00007f567cfe4000]
   java.lang.Thread.State: RUNNABLE
	at java.net.SocketInputStream.socketRead0(Native Method)
	at java.net.SocketInputStream.read(SocketInputStream.java:150)
	at java.net.SocketInputStream.read(SocketInputStream.java:121)
	at java.io.BufferedInputStream.read1(BufferedInputStream.java:284)
	at java.io.BufferedInputStream.read(BufferedInputStream.java:345)
	- locked <0x00000007aa3ec760> (a java.io.BufferedInputStream)
	at sun.net.www.MeteredStream.read(MeteredStream.java:134)
	- locked <0x00000007aa3eedd8> (a sun.net.www.MeteredStream)
	at java.io.FilterInputStream.read(FilterInputStream.java:133)
	at sun.net.www.protocol.http.HttpURLConnection$HttpInputStream.read(HttpURLConnection.java:3322)
	at sun.net.www.protocol.http.HttpURLConnection$HttpInputStream.read(HttpURLConnection.java:3315)
	at org.apache.commons.io.input.ProxyInputStream.read(ProxyInputStream.java:77)
	at hudson.model.UpdateCenter$UpdateCenterConfiguration.download(UpdateCenter.java:766)
	at hudson.model.UpdateCenter$DownloadJob._run(UpdateCenter.java:1138)
	at hudson.model.UpdateCenter$InstallationJob._run(UpdateCenter.java:1298)
	at hudson.model.UpdateCenter$DownloadJob.run(UpdateCenter.java:1116)
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
	at hudson.remoting.AtmostOneThreadExecutor$Worker.run(AtmostOneThreadExecutor.java:104)
	at java.lang.Thread.run(Thread.java:745)

[FIXED JENKINS-25234] Improve readability of the 'schedule build' tooltip

SCM._for is actually @CheckForNull.

[FIXED JENKINS-20606] Wrong spelling in Russian localization

Updating SingleFileSCM to nondeprecated overload of checkout.

This allows @Initializer to have parameter injection

This demonstrated that we actually need to call EnvVars.overrideAll explicitly (as CoreWrapperStep already did).