feature request: handle online check behind captive portals

[thuehn]

In order to check http access through captive portals, we would need to enahce our online check by:
-max-redirects with curl does not detect all CPs
-wget / uclient-fetch can not detect CPs
-pattern matching per host (e.g.: host = http://www.google.de, pattern = '<title>Google</title>')