Clean up FilePath. Extract one method out and ensure closing of input stream after copy

Fixes JENKINS-13649

[FIXES JENKINS-13649] As FilePath(FilePath,String) expects multi-segment relative paths, we should ensure that the multiple segments are using the correct separator character for the remote OS

we shouldn't change the definition of it.

Instead, I'm creating a new tag.

... to fix a possible race condition.

[JENKINS-13536] I think the proper thing to do is to use the Accept header, not the Content-Type header.

When a client uses the buildWithParameters API and request a json output, return the job definition as a json string instead of doing a redirect on the job page.

[FIXED JENKINS-7806] changed wording of "slave back online" button