[FIXED JENKINS-6153] Add description for label atoms

[FIXED JENKINS-19764] Ensure parent dir of new builds dir exists.

Since ConsistentHash is written as a mutable class, I think a better way to fit the existing design is to provide a bulk add operation.
It already has several methods to that end, so I just added one more that's needed for our purpose.

Incorrect link to master node on label page

invoking add(Node, replica) for all nodes rebuild the internal Table many times. Using a builder, Table is only created (and sorted) once
also avoid creating a fresh new Hash object (java 8 lambdas, I miss you)

Ubuntu (at least as of 12.04) has the default umask 022, which made some
users nervous. Quoting its /etc/login.defs below, which explains its
historical origin:

  UMASK is the default umask value for pam_umask and is used by
  useradd and newusers to set the mode of the new home directories.
  022 is the "historical" value in Debian for UMASK
  027, or even 077, could be considered better for privacy
  There is no One True Answer here : each sysadmin must make up his/her
  mind.

It does seem to me that a bit more restrictive default is sensible,
so this change introduces /etc/default/jenkins parameter that sets the
default umask to 027 to prevent "others" from seeing files.

Not that keys and other sensitive files are protected anyway, so it is
not the case that the privacy of Jenkins data files have been vulnerable
prior to this change.

java.lang.NullPointerException: null
	at hudson.model.AbstractBuild.setWorkspace(AbstractBuild.java:296)
	at hudson.model.ProjectTest.testCheckout(ProjectTest.java:423)

[FIXED JENKINS-23522] Defend against stack overflows when a listener (unnecessarily) called super from a deprecated method.
Fixes #1295 a little differently.

If we are running on a case-insensitive FS, the test makes no sense because migration would never even be attempted to begin with.

For some reason IE10 craches when document.getElementsByName is called and there is an element with a name attribute but no value. In this case a button had a name attribute set with an empty string as the value.