discontinue readavailable()

This is really hard function to use correctly, since the only correct usage of this function is in a stream reader:
```julia
buf = Uint8[]
while !eof(s)
  bytes = readavailable(s)
  append!(buf, s)
  if enough(buf)
     f(buf)
     empty!(buf)
  end
end
f(buf)
```

However, essentially all of the code I see posted to blogs and mailing lists to use this function are incorrectly defined, and subject to race conditions and the badly-defined behavior of this function (that and probably will stop working as soon as you try to bundle it into your application and output a decent amount of information, or forget that stdout is buffered, or run into a bit about latency)

I have seen users resort to it in the following situation, when users can't understand why `readall` hangs:
```julia
(outRead, outWrite) = redirect_stdout()

print("Test")
print("ing")
 
close(outWrite)
 
data = readavailable(outRead)
 
close(outRead)
```
(copied from: http://thenewphalls.wordpress.com/2014/03/21/capturing-output-in-julia/)

however, the answer to their quandary is simple: `outWrite` is only one of the handles to the stdout. until you call `redirect_stdout` again, julia still has a second handle open (on fd 2) for use by `c` functions

related: https://github.com/JuliaLang/julia/issues/7022
how about:
"readavailable is discontinued. please choose from the alternative I/O functions, such as readall(), readbytes(), nb_available(), and !eof(), depending on your intended usage"