Make sure that we don't require Jenkins.getInstance() on CLICommand, since it can run on slaves.

JENKINS-13185: Fix Computer.getHostName() to return the host.name value from the slave rather than null

Fix for JENKINS-13185: If there is a fallback host.name specified, it should return that and not null.

This has a downside that we waste cache in user browser, especially for
plugin developers, but this provides the simplest migration for plugins,
and the same code works for pre 1.478 and post 1.478.

When Reloading Configuration from Disk in Jenkins, all swarm clients
were in an unusable state afterwards (eg. missing their labels).
This happened because Swarm Clients were not written into the
configuration file, and got removed when reloading the config.

This patch re-adds all previously existing swarm clients after
reloading the config file.

Earlier Jenkins advertised being in port 80 regardless
what port it was really listening.

Also added registration to service _http._tcp which makes
Jenkins appear in Bonjour bookmarks of the Safari browser.

Fix NPE in Launcher.decorateByPrefix

fix tabs

exposing the properties from the base class so that consumers can rely on them regardless of the actual subtype

fixes: JENKINS-12889 Jenkins "Workspace" presents "Run a build" even when Builds are Disabled

Add CANCEL permission that can be granted independently from BUILD perm.

add http header for under firewall enviroment and proxy to cache
Thanks