Set proper content types and work with temporary cache file fragments

Set proper content types and work with temporary cache file fragments

With WebKit 1.1.11 and newer, we can sniff the content type which
is needed in some cases to load for instance Flash videos.

We are writing chunks into temporary files and commit completed
files to the cache, so that we don't end up serving partial files.

Old cache entries are removed before updating.

Length detection is improved.

File creation moved from got-chunk to got-headers.