Make hexl scrolling commands work more like the normal ones

* lisp/hexl.el (hexl-scroll-down):
(hexl-scroll-up): Heed `next-screen-context-lines' (bug#7031).

etc/NEWS

@@ -1370,6 +1370,12 @@ This new command (bound to 'C-c C-l') regenerates the current hunk.
 
 ** Miscellaneous
 
+---
+*** 'hexl-mode' scrolling commands now heed 'next-screen-context-lines'.
+Previously, 'hexl-scroll-down' and 'hexl-scroll-up' would scroll
+up/down an entire window, but they now work more like the standard
+scrolling commands.
+
 ---
 *** Errors in 'kill-emacs-hook' no longer prevent Emacs from shutting down.
 If a function in that hook signals an error in an interactive Emacs,

lisp/hexl.el

@@ -724,7 +724,8 @@ With prefix arg N, puts point N bytes of the way from the true beginning."
   (setq arg (if (null arg)
                 (- (window-height)
                    1
-                   (if ruler-mode 1 0))
+                   (if ruler-mode 1 0)
+                   next-screen-context-lines)
               (prefix-numeric-value arg)))
   (hexl-scroll-up (- arg)))
 
@@ -735,7 +736,8 @@ If there's no byte at the target address, move to the first or last line."
   (setq arg (if (null arg)
                 (- (window-height)
                    1
-                   (if ruler-mode 1 0))
+                   (if ruler-mode 1 0)
+                   next-screen-context-lines)
               (prefix-numeric-value arg)))
   (let* ((movement (* arg 16))
 	 (address (hexl-current-address))