Reading/writing from/to a pipe
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   The "procbuf" class is a GNU extension.  It is derived from
`streambuf'.  A `procbuf' can be "closed" (in which case it does
nothing), or "open" (in which case it allows communicating through a
pipe with some other program).

 - Constructor:  procbuf::procbuf ()
     Creates a `procbuf' in a "closed" state.

 - Method: procbuf* procbuf::open (const char *COMMAND, int MODE)
     Uses the shell (`/bin/sh') to run a program specified by COMMAND.

     If MODE is `ios::in', standard output from the program is sent to
     a pipe; you can read from the pipe by reading from the `procbuf'.
     (This is similar to `popen(COMMAND, "r")'.)

     If MODE is `ios::out', output written written to the `procbuf' is
     written to a pipe; the program is set up to read its standard
     input from (the other end of) the pipe.  (This is similar to
     `popen(COMMAND, "w")'.)

     The `procbuf' must start out in the "closed" state.  Returns
     `*this' on success, and `NULL' on failure.

 - Constructor:  procbuf::procbuf (const char *COMMAND, int MODE)
     Calls `procbuf::open (COMMAND, MODE)'.

 - Method: procbuf* procbuf::close ()
     Waits for the program to finish executing, and then cleans up the
     resources used.  Returns `*this' on success, and `NULL' on failure.

 - Destructor:  procbuf::~procbuf ()
     Calls `procbuf::close'.