• add aio_truncate, aio_chmod, aio_chown, aio_utime (linux
  successfully demonstrated that you can block on futimes...).
  
• allow undef as fileoffset for aio_read/write and use read/write(2)
  internally (useful for sockets or O_APPEND handles).
  
• allow undef for length in aio_write.
  
• negative dataoffsets work as expected now in aio_read/aio_write.
  
• use NV instead of UV for 32 bit perls and file offsets, as NVs
  have a larger range then.