Re: call-with-input-pipe, call-with-output-pipe: allow list argument

	The current implementation of call-with-input-pipe and
	call-with-output-pipe could be further extended to allow for
	filters to be run from Scheme48.  In particular, it'll allow for
	pipes to be constructed by pure Scheme48 means, like:

;; rough equivalent of Shell's cal_nl_output="$(cal | nl)"
foo> (define cal+nl-output
       (call-with-input-pipe '("cal")
         (lambda (cal-output)
           (call-with-input-pipe '("nl")
	     read-all/string
             cal-output))))	; NB: redirecting nl's stdin
; no values returned
foo> (display cal+nl-output)
     1	    October 2008    
     2	Su Mo Tu We Th Fr Sa
     3	          1  2  3  4
     4	 5  6  7  8  9 10 11
     5	12 13 14 15 16 17 18
     6	19 20 21 22 23 24 25
     7	26 27 28 29 30 31   
     8	                    
#{Unspecific}
foo> 

	The change is as follows.

Extend `call-with-mumble-pipe' to allow an optional `port' argument.
(call-with-mumble-pipe): Extended the function returned to accept an
optional port argument to allow for running programs in ``filter'' mode.
(call-with-input-pipe): Extended likewise.
(call-with-output-pipe): Likewise.

--- pipe.scm.~3~	2008-10-21 00:14:14.000000000 +0700
+++ pipe.scm	2008-10-22 21:48:16.000000000 +0700
@@ -7,7 +7,7 @@
 ; end of the pipe and COMMAND gets the output end.
 
 (define (call-with-mumble-pipe input?)
-  (lambda (command proc)
+  (lambda (command proc . may-be-port)
     (call-with-values open-pipe
       (lambda (input-pipe output-pipe)
 	(let ((pid (fork)))
@@ -27,12 +27,19 @@
 	       (lambda ()
 		 #f)
 	       (lambda ()
+		 (define port
+		   (and (not (null? may-be-port))
+			(car may-be-port)))
 		 (if input?
-		     (remap-file-descriptors! (current-input-port)
+		     (remap-file-descriptors! (or
+					       port
+					       (current-input-port))
 					      output-pipe
 					      (current-error-port))
 		     (remap-file-descriptors! input-pipe
-					      (current-output-port)
+					      (or
+					       port
+					       (current-output-port))
 					      (current-error-port)))
 		 (cond ((list? command)
 			(apply exec command))