[BUGFIX] Hardcoded fileadmin directory in Import-Export

$fileAdminDir is hardcoded in t3lib\class.t3lib_softrefproc.php
and typo3\sysext\impexp\class.tx_impexp.php
$TYPO3_CONF_VARS['BE']['fileadminDir'] should be used instead

Change-Id: I9f381507a762352ce6a555134534481e4df61f92
Releases: 4.5, 4.6, 4.7, 6.0
Resolves: #41120
Reviewed-on: http://review.typo3.org/15378
Reviewed-by: Wouter Wolters
Reviewed-by: Philipp Gampe
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer
Reviewed-by: Oliver Hader
Tested-by: Oliver Hader