Don't keep temp CReadState on CurlFile::Seek

Apparently there are a lot of servers around that limit to two
concurrent connections.  This also eliminates a potential leak of the
temporary CReadState should the early return be taken.
Try CurlFile::Seek without multisession on failure
Minor refactor of CurlFile::Seek
Merge pull request #4546 from t-nelson/curl_no_more_than_two_in_multi

Curl multi-session fixes