[BUGFIX] Placeholder JavaScript handling is active for Non-IE browsers

The jsfunc.placeholder.js file was introduced to have a fallback
for Internet Explorer on the HTML5 placeholder feature. However,
this handling is currently active in all browsers and results in
broken NULL values for textarea, since textarea don't have a
human readable representation like input fields.

Fixes: #43183
Releases: 6.0
Thus, only activate the handling in Internet Explorer.

Change-Id: Ie27c8e8544fa99cad14f3a74fa960524cc7e834a
Reviewed-on: http://review.typo3.org/16644
Reviewed-by: Oliver Hader
Tested-by: Oliver Hader