bindings: allow using '/' to start a search in the help viewer

And allow 'N' for searching the next occurrence.  Add the
same keystrokes to the file browser too, for consistency.

src/browser.c

@@ -490,7 +490,11 @@ functionptrtype parse_browser_input(int *kbinput)
 		return do_enter;
 	    case 'W':
 	    case 'w':
+	    case '/':
 		return do_search;
+	    case 'N':
+	    case 'n':
+		return do_research;
 	}
     }
     return func_from_key(kbinput);

src/help.c

@@ -574,6 +574,13 @@ functionptrtype parse_help_input(int *kbinput)
 		return do_page_down;
 	    case '-':
 		return do_page_up;
+	    case 'W':
+	    case 'w':
+	    case '/':
+		return do_search;
+	    case 'N':
+	    case 'n':
+		return do_research;
 	    case 'E':
 	    case 'e':
 	    case 'Q':