[BUGFIX] Transfer curlProxyServer to new HTTP settings

Splits up the old curlProxyServer correctly, stripping "http://" and
potentially trailing slashes.

Resolves: #45834
Releases: 6.3, 6.2, 6.1
Change-Id: Ibfefb927b5fd3fe900170870e1093027d4fe34b5
Reviewed-on: http://review.typo3.org/31880
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>