Misc cleanups

[JENKINS-59580] Fix WindowsOSProcess.getEnvironmentVariables returning null

Use type inference instead of specifying it

Fix ProcessTreeKillerTest#doNotKillProcessWithCookie to work on Windows

Spotbugs: changed order for compare instead of negating result 

Spotbugs: Replaced inefficient Number constructor with static valueOf

Trailing whitespace, wrong indentation

Javadoc for ProcessTree#killAll says that "either of the parameter can be null",
but it wasn't true for proc.

[JENKINS-58993] do not allow save() to be called before we have loaded the config

OSProcess.getEnvironmentVariables javadoc clearly states that method will return empty map if obtaining of environment variables failed.

* Use different icons for computers that are temporarily offline

* [JENKINS-59283] Refine the icons semantics

* extracted duplicate code into helper method

* merged two identical switch statements

* Update cli/src/test/java/hudson/cli/PrivateKeyProviderTest.java
Co-Authored-By: NMatt Sicker <boards@gmail.com>

* Apply suggestions from code review
Co-Authored-By: NOleg Nenashev <o.v.nenashev@gmail.com>

* Update cli/src/test/java/hudson/cli/PrivateKeyProviderTest.java
Co-Authored-By: NTim Jacomb <t.jacomb@kainos.com>

* renamed method as suggested by timja

* Introduced hudson.TcpSlaveAgentListener.getAdvertisedHost()

* review feedback

* better error message

* Let remoting handle missing channels

* Bump minimum remoting version to 3.14

* [JENKINS-59514] Use POST instead of RequirePOST for form submissions

* More form submission URLs

* More form submission endpoints

This limit:
1. Isn't justified by anything
2. Cannot be configured by users
3. Doesn't properly interact with nontrivial HTML markup. For example, it sums text in <option> elements
   even though they do not actually take up more visual space on page and instead only longest <option> matters

* Update PULL_REQUEST_TEMPLATE.md

Removed a defunct hyperlink and placed an intuitive directive in its place.

* Updated PULL_REQUEST_TEMPLATE.md to include comment

* Updated PULL_REQUEST_TEMPLATE.md to include lts backporting

removed powermock dependency because it was not used in cli

Minor typo fix on line 94