* [JENKINS-37332] - Improve diagnostics of non-closed streams during reading of the manifests in PluginManager

* [JENKINS-37332] - Leakless processing of JarUrlConnection during Manifest parsing

* [JENKINS-37332] - Also implement leak-safe method for retrieving file modification date

* [JENKINS-37332] - Add spotcheck methods for manifest file access + Javadoc

* [JENKINS-37332] - Also test multi-line and empty attributes in the test

FindBugs: Suppress DM_GC in hudson.Util#pauseBetweenDeletes(), because it's a desired behavior for custom setting

* [JENKINS-35570] List of computer's names extracted to Util

* [JENKINS-34854] - Migrate missing Integer and Boolean properties to SystemProperties

* [JENKINS-34854] - API methods for Long variables

* [JENKINS-34854] - Migrate Long settings to SystemProperties

* [JENKINS-34854] - Leftover change (I should use Save All)

* [JENKINS-34854] - Some fixes in Long methods Javadoc and log messages

Simplify the code now the minimum version is Java 7.

Renamed system property from ...deletionMax to ...maxFileDeletionRetries (as requested during code review).

FIXED JENKINS-15331 by changing Util.deleteContentsRecursive, Util.deleteFile and Util.deleteRecursive so that they can retry failed deletions.
The number of deletion attempts and the time it waits between deletes are configurable via system properties (like hudson.Util.noSymlink etc).
Util.DELETION_MAX is set by -Dhudson.Util.deletionMax.  Default is 3 attempts.
Util.WAIT_BETWEEN_DELETION_RETRIES is set by -Dhudson.Util.deletionRetryWait.  Defaults is 100 milliseconds.
Util.GC_AFTER_FAILED_DELETE is set by -Dhudson.Util.performGCOnFailedDelete.  Default is false.

Added unit-tests for new functionality.

* 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)

Signed-off-by: NOleg Nenashev <o.v.nenashev@gmail.com>

The change adds annotations for all methods inside the class. The initial idea - enforce checks of symlink methods to prevent NPEs on Windows platforms.
Signed-off-by: NOleg Nenashev <o.v.nenashev@gmail.com>

Retrying 3 times while waiting for 100 millis between retries (at
recreating symlink after "deleting" it). Ends up throwing FAE exception
if thrown for a 4th time in a row - as not much can be done at that
point. A warning is logged in that case, prior to throwing the last FAE.

Given these <variable,value> mappings: <A,a> and <A.B,a-b>, $A.B would
evaluate to $a.B, as it currently does, and ${A.B} to a-b instead of the
current ${A.B}.

Existing ${A.B}-like references will break (not evaluate to ${A.B}) if
there actually is an A.B variable defined, which I think is very
unlikely.

[[JENKINS-16301] Use a buffered input stream to get somewhat better performance also from Maven archiving.
Introducing Util.getDigestOf(File) as a convenience.

[FIXED JENKINS-18178] Reverting an inessential part of the fix of JENKINS-16301 since it broke some Maven builds.
Apparently the Maven 2 process factory fails to adequately insulate the Maven process from Jenkins library dependencies;
if you specify a build extension that can override dependencies used by Jenkins core classes!
A real fix would involve changing class loader delegation, since presumably similar bugs could still occur.

[JENKINS-16301] Replace impl of getDigestOf with standard (Commons Codec) DigestUtils.md5Hex, for simplicity.

[FIXED JENKINS-17681] Do not try to call resolveSymlink on a nonexistent link as you will just get an IOException, breaking peephole permalinks in some Windows environments.

On Chrome (and possibly others), an inline element like span doesn't enlarge
its size to cover its content, so the 1px place holder wasn't taking effect.

To ensure the minimum height of 16px, make it an inline-block element. I'm
not entirely sure how an inline-block element with no fixed width would behave,
but with a bit of casual testing, the text inside seems to wrap OK.

As I mentioned in the comment, I doubt if this makes any behaviour change, but this seems like a sensible defensive measure that wouldn't hurt.

Don't break Windows XP, fall back gracefully to no-op.

- ID_FORMATTER is time zone sensitive, so comparing against # of
  milliseconds since the epoch won't be portable.

- resolveSymlink can return a relative path.