required dependencies; this may still have some issues in unknown cases
such as a plugin disabled, then re-enabled without restarting Jenkins
but it fixes one of the primary cases

Also adds some API restrictions for JENKINS-33803.

UpdateSite, when reading plugin metadata

Also ... unbundling of plugins from Jenkins core.

* moved to jenkins.util.SystemProperties
* consistent naming getString/getInteger/getBoolean
* improved code formatting
Improved JavaDoc

Accomplished by centralizing calls to System.getProperty(String) and related into new file SystemProperties.java.
There, we first check for existence of system property; if not, we look for property in context.xml
This is done for "application" properties (like hudson.DNSMultiCast.disabled) but not for java properties (like user.name)

(cherry picked from commit df9c92e4)

(cherry picked from commit 1ac77750)

Conflicts:
	core/src/main/java/hudson/model/DownloadService.java
	core/src/main/java/hudson/model/UpdateSite.java

Since sorting of plugins in the update manager is based on displayName
this name should not have a common prefix otherwise it's hard to find
a given plugin name.

[FIXED JENKINS-19739] When installing a plugin and the needed dependencies have compatibility issues, warn the user

[FIXED JENKINS-19736] Installing a plugin with optional dependencies doesn't upgrade the optional dependencies when needed

Due to a confusion between 0 and -1. Easier and clearer to use 0 as the “uninitialized” value.

I had to tweak UpdateSite code because JSONObject.fromObject(String)
became more strict and no longer allows extra garbage after the closing
'}'

Removing old static field, which was not friendly to tests, especially multithreaded runners.
Cleaned up existing tests to use updateDirectly rather than POSTing raw JSON.

Correctly support direct updates of html update sites and other issues pointed out by jglick in https://github.com/jenkinsci/jenkins/pull/692

[FIXED JENKINS-15889] Saving the update center list after the metadata has been fetched results in the metadata being persisted twice

[FIX JENKINS-10569] When installing plugins with overlapping dependencies, Jenkins downloads the duplicate plugins multiple times