Added one more property:

(os.stdio symbol symbol symbol)

Three symbols in this order:

* File type of standard input (stdin, Unix file descriptor 0)
* File type of standard output (stdout, Unix file descriptor 1)
* File type of standard error (stderr, Unix file descriptor 2)

Each symbol shall give the type of the port in the current environment.
The following choices are standard:

* terminal
* device
* socket
* file
* pipe
* unknown

Knowing the types can be useful for debugging unusual I/O behavior in a
particular environment.

Examples:

(os.stdio device pipe pipe)
(os.stdio socket terminal file)