util: copy_bytes: Multiple changes

util: copy_bytes: Multiple changes

 - Use max possible buffer size for sendfile()

   The fewer we call sendfile() the better (performance)

 - Provide read()/write() fallback when sendfile will
   not work (eg. copying data from pipe to pipe)

 - Add third argument with number of copied bytes. This is
   to avoid mixing error code (negative) with transferred size.

 - Make API available for rest of codebase

Change-Id: If80feb578b01fb715008a20e7c000c5ede9b62a1