[FIXED JENKINS-19251] Use hashes rather than modification time to determine if Maven agent JARs are up to date.

- Adds an option to the Global Security screen to hide the remember me checkbox
- A crafty user could use their browser to re-insert the form field and thereby get Jenkins to set the
  remember_me cookie on their browser, but this option should cover the 99.9% case.

This reverts commit 647695e2.

This reverts commit 647695e2.

[FIXED JENKINS-19613] 49c731fc made PROXY_HEADER static under the assumption that XStream would treat that like a missing field.
Unfortunately it threw an error instead, breaking startup; fixed by renaming the constant.

- This is an initial version of the feature.
- The information about slave names is still exposed via label autocomplete

1. Use AtomicFileWriter on the hypothesis that an exception during save() caused truncation.
2. Improve logging from the cleanup thread since that may be related to the root cause.
3. Regardless of the cause, recover more gracefully by deleting any truncated file rather than throwing the error up.

Instead it was being thrown in a Future that the master did not wait for:
the writing side failed with a closed pipe exception, and this meaningless error was thrown up.
Fixing by waiting for the reading side first, so any error there will be thrown up.
(If there was _also_ an error on the writing side, we would miss it, but typically this will just be the closed stream.)