utils: 'elf-file?' and 'ar-file?' return #f for directories.

This avoids uncaught exceptions when the 'strip' phase would call these
procedures on symlinks to directories, such as 'lib/terminfo' in
ncurses (see <http://hydra.gnu.org/build/167310/nixlog/1/tail-reload>.)

* guix/build/utils.scm (file-header-match): Catch 'system-error', and
  return #f upon EISDIR.

guix/build/utils.scm

@@ -122,7 +122,13 @@ (define (get-header)
           (get-bytevector-n port len))
         #:binary #t #:guess-encoding #f))
 
-    (equal? (get-header) header)))
+    (catch 'system-error
+      (lambda ()
+        (equal? (get-header) header))
+      (lambda args
+        (if (= EISDIR (system-error-errno args))
+            #f                                    ;FILE is a directory
+            (apply throw args))))))
 
 (define %elf-magic-bytes
   ;; Magic bytes of ELF files.  See <elf.h>.