ecere/net/HTTPFile: (#997) Multiple fixes and improvements

- Requesting HEAD only on Open() and GET on first Read() contents
- Added support for Content-Type and Content-Disposition
- Using List container to hold connections with reference counting fixes
- Fixed hangs upon closing the file caused by improper tracking of position
- Fixed hangs caused by improper Locking/Unlocking of connectionMutex
- Added Seek() support for small files fitting within the buffer