Work around flash fs toggle issues in webkit (#126, #389)

- detect troublesome browsers correctly (webkit, windows Safari)
- always resume current clip in playlist
- over rtmp seek to current position

The latter cannot be approximated reliably with progressive download
because of issue #472 - although the video is buffered to the current
position there are problems with seeking on start.
Simplify conditional for fs resume pos detection (#126)
Merge pull request #473 from blacktrash/flash-webkit-fs-resume

Work around flash fs toggle issues in webkit (#126, #389)