HTTPFile: use IO.sysread for better performance

HTTPFile: use IO.sysread for better performance

IO.sysread is more GC-friendly than IO.read because it does not
have to allocate additional userspace buffers.  Userspace read
buffering is redundant nowadays with modern kernels (especially
in Linux 2.6 and almost always a performance hit).

Additionally, use an underdocumented feature of both IO.sysread
and IO.read that allows it to reuse the existing buffer.
Further reducing garbage collector overhead for large files.