[FIXED JENKINS-19926] Add only new env. variables

Previous patch (3eea39648943583a530f47f69f93737266537d9d) broke usescases when evn. variable overwrites already existing one
like PATH = $PATH:/my/path
Merge pull request #970 from vjuranek/JENKINS-19926

[FIXED JENKINS-19926] Add only new env. variables
[FIXED JENKINS-20071] Upgrade bundled versions of credentials and ssh-slaves so we can assume <c:select/> available
[JENKINS-18629] Noting retroactively.
[FIXED JENKINS-20085] Overflow in Jenkins.globalIota. Simpler and safer to use an AtomicLong.
[FIXED JENKINS-16936] Added SecureRequester extension point.
[FIXED JENKINS-20080] Build Button fails for jobs that require "Build with Parameters"
if restart thread dies for any reasons, record that fact
Jenkins!
This version has a better error diagnostics
Merge branch 'master' of github.com:jenkinsci/jenkins
testTriggerJob has been unstable recently, probably because of executor-ondemand branch changes.
getItem(String,ItemGroup) Javadoc failed to mention the fallback to getItemByFullName.
Removing bad hyperlink from noWorkspace.properties#text.
Merge branch 'fix-build-icon' of github.com:olivergondza/jenkins into olivergondza-fix-build-icon
[JENKINS-20080] Noting that pull #975 was merged.
Updated Japanese localization
[maven-release-plugin] prepare release jenkins-1.536
[maven-release-plugin] prepare for next development iteration
updated changelog for release