R6RS: Have get-char', get-line', etc. raise an `&i/o-decoding-error'.

* module/rnrs/io/ports.scm (&i/o-decoding): New error condition type.
  (with-i/o-decoding-error): New macro.
  (get-char, get-datum, get-line, get-string-all, lookahead-char): Use
  it.

* test-suite/tests/r6rs-ports.test ("8.2.6  Input and output
  ports")["transcoded-port [error handling mode = raise]"]: Use `guard'
  and `i/o-decoding-error?'.

test-suite/tests/r6rs-ports.test

@@ -22,6 +22,7 @@
   #:use-module (srfi srfi-1)
   #:use-module (srfi srfi-11)
   #:use-module (rnrs io ports)
+  #:use-module (rnrs exceptions)
   #:use-module (rnrs bytevectors))
 
 ;;; All these tests assume Guile 1.8's port system, where characters are
@@ -542,13 +543,9 @@
                                 (error-handling-mode raise)))
            (b  (open-bytevector-input-port #vu8(255 2 1)))
            (tp (transcoded-port b t)))
-      ;; FIXME: Should be (guard (c ((i/o-decoding-error? c) #t)) ...).
-      (catch 'decoding-error
-        (lambda ()
-          (get-line tp)
-          #f)
-        (lambda _
-          #t))))
+      (guard (c ((i/o-decoding-error? c)
+                 (eq? (i/o-error-port c) tp)))
+        (get-line tp))))
 
   (pass-if "transcoded-port [error handling mode = replace]"
     (let* ((t  (make-transcoder (utf-8-codec) (native-eol-style)
@@ -559,4 +556,5 @@
 
 ;;; Local Variables:
 ;;; mode: scheme
+;;; eval: (put 'guard 'scheme-indent-function 1)
 ;;; End: