Added new `port-position' procedure. On a port created using

Added new `port-position' procedure.  On a port created using
`open-output-string', e.g. through `with-output-to-string', this
procedure returns the current position in the output buffer.  For
example:

  1 ]=> (define foo)

  ;Value: foo

  1 ]=> (with-output-to-string
	  (lambda ()
	    (write-string "foo")
	    (set! foo (port-position (current-output-port)))
            (write-string "bar")))
  ;Value 1: "foobar"

  1 ]=> foo

  ;Value: 3

  1 ]=>