Extend fio Lua API

In order to use fio in conjunction with ibuf, we need to extend read(),
pread(), write(), pwrite() so that they can take a C buffer instead of
a Lua string. The syntax is as follows:

  read(size) -> str
  read(buf, size) -> len

  pread(size, offset) -> str
  pread(buf, size, offset) -> len

  write(str)
  write(buf, size)

  pwrite(str, offset)
  pwrite(buf, size, offset)

See #2755