Fix jenkins restart on Linux after a JRE system update

Daemon.getCurrentExecutable() use /proc/PID/exe to retrieve the used JRE. This link is broken when the target changes (ie after a JRE system update) because the kernel appends ' (deleted)' to the symlink. This patch uses the first argument of the command line (in /proc/PID/cmdline) instead.

[JENKINS-22631] The wrong (newer) build was sometimes being marked for fixing, depending on arbitrary directory listing order.

Trying to make SocketTimeoutException’s (as originally noted in 3ec7e562) not turn into test errors. (Works only in JenkinsRule, not HudsonTestCase.) Still have no clue as to cause.

[FIXED JENKINS-22631] Do not just warn about multiple builds with the same number; note this in the official problem list, and offer to fix it.

[FIXED JENKINS-21622] Release workspace leases (and avoid @2) even if post-build actions throw LinkageError.

[JENKINS-22583] Noting merge of #1182.
Conflicts:
	changelog.html

Taking advantage of https://github.com/stapler/stapler-adjunct-codemirror/issues/1 for new CodeMirror.

[FIXED JENKINS-22607] Avoid loading recent builds when deletion of builds older than some date has been requested.