detect and consume byte-order marks for textual ports

* libguile/ports.h:
* libguile/ports.c (scm_consume_byte_order_mark): New procedure.

* libguile/fports.c (scm_open_file): Call consume-byte-order-mark if we
  are opening a file in "r" mode.

* libguile/read.c (scm_i_scan_for_encoding): Don't do anything about
  byte-order marks.

* libguile/load.c (scm_primitive_load): Add a note about the duplicate
  encoding scan.

* test-suite/tests/filesys.test: Add tests for UTF-8, UTF-16BE, and
  UTF-16LE BOM handling.

test-suite/tests/filesys.test

@@ -1,6 +1,6 @@
 ;;;; filesys.test --- test file system functions -*- scheme -*-
 ;;;; 
-;;;; Copyright (C) 2004, 2006 Free Software Foundation, Inc.
+;;;; Copyright (C) 2004, 2006, 2013 Free Software Foundation, Inc.
 ;;;; 
 ;;;; This library is free software; you can redistribute it and/or
 ;;;; modify it under the terms of the GNU Lesser General Public
@@ -17,6 +17,8 @@
 ;;;; Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
 
 (define-module (test-suite test-filesys)
+  #:use-module (ice-9 rdelim)
+  #:use-module (ice-9 binary-ports)
   #:use-module (test-suite lib)
   #:use-module (test-suite guile-test))
 
@@ -127,3 +129,58 @@
 
 (delete-file (test-file))
 (delete-file (test-symlink))
+
+(let ((s "\ufeffHello, world!"))
+  (define* (test-encoding encoding #:optional (ambient "ISO-8859-1"))
+    (with-fluids ((%default-port-encoding ambient))
+      (let* ((bytes (catch 'misc-error
+                      (lambda ()
+                        (call-with-values open-bytevector-output-port
+                          (lambda (port get-bytevector)
+                            (set-port-encoding! port encoding)
+                            (display s port)
+                            (get-bytevector))))
+                      (lambda args
+                        (throw 'unresolved))))
+             (name (string-copy "myfile-XXXXXX"))
+             (port (mkstemp! name)))
+        (put-bytevector port bytes)
+        (close-port port)
+        (let ((contents (call-with-input-file name read-string)))
+          (delete-file name)
+          contents))))
+
+  (pass-if "UTF-8"
+    (equal? (test-encoding "UTF-8")
+            "Hello, world!"))
+
+  (pass-if "UTF-16BE"
+    (equal? (test-encoding "UTF-16BE")
+            "Hello, world!"))
+
+  (pass-if "UTF-16LE"
+    (equal? (test-encoding "UTF-16LE")
+            "Hello, world!"))
+
+  (pass-if "UTF-8 (ambient)"
+    (equal? (test-encoding "UTF-8" "UTF-8")
+            "Hello, world!"))
+
+  (pass-if "UTF-8 (UTF-16 ambient)"
+    (equal? (test-encoding "UTF-8" "UTF-16")
+            "Hello, world!"))
+
+  ;; Unicode 6.2 section 16.8:
+  ;;
+  ;; For compatibility with versions of the Unicode Standard prior to
+  ;; Version 3.2, the code point U+FEFF has the word-joining semantics
+  ;; of zero width no-break space when it is not used as a BOM.  [...]
+  ;;
+  ;; Where the byte order is explicitly specified, such as in UTF-16BE
+  ;; or UTF-16LE, then all U+FEFF characters -- even at the very
+  ;; beginning of the text -- are to be interpreted as zero width
+  ;; no-break spaces.
+  ;;
+  (pass-if "UTF-16LE (ambient)"
+    (equal? (test-encoding "UTF-16LE" "UTF-16LE")
+            "\ufeffHello, world!")))