Bug 24985 use $wgTmpDirectory when available

Bug 24985 use $wgTmpDirectory when available

We had two way to get a temporary directory:

 - $wgTempDirectory: more or less stable accross sessions
 - wfTempDir(): set through environnement variable and could potentially
   vary from a session to another one thanks to tempnam()

This patch makes wfTempDir() to always use the global $wgTempDirectory
first when it is available. Thus explicitly overriding tempnam() or any
environnement variable such as TMP or TEMP.

Hence, people who don't have access to a system wide directory
specificed by their environnement (such as /tmp) can specify an
alternative straight from the MediaWiki configuration.

The patch remove references to $wgTmpDirectory and replace them with
calls to wfTempDir().  Make wfTempDir() use $wgTmpDirectory first.

The default setting of $wgTmpDirectory was removed in favor of having it
initialized through Setup.php by calling wfTempDir.

Note: this may also address Bug 36475 - Generating thumbnails does not
work when there is no access to /tmp

Change-Id: Ifdc79e9c5d95f978025b237a5eeb95fd75092f46