browser: keep the highlight in the same spot or column, when possible

When doing a PageUp or PageDown in the browser, don't move the highlight
to the first line in the same column, but keep it in the same relative
position of the screen.  If we're already on the first or last page,
move the highlight to the first or last line, but keep it in the same
column.  If we're already on the first or last line, only then move it
to the first or last entry.

src/browser.c

@@ -187,14 +187,20 @@ char *do_browser(char *path, DIR *dir)
 	    /* Search for another filename. */
 	    do_fileresearch();
 	} else if (func == do_page_up) {
-	    if (selected >= (editwinrows + fileline % editwinrows) * width)
-		selected -= (editwinrows + fileline % editwinrows) * width;
-	    else
+	    if (selected < width)
 		selected = 0;
+	    else if (selected < editwinrows * width)
+		selected = selected % width;
+	    else
+		selected -= editwinrows * width;
 	} else if (func == do_page_down) {
-	    selected += (editwinrows - fileline % editwinrows) * width;
-	    if (selected > filelist_len - 1)
+	    if (selected + width >= filelist_len - 1)
 		selected = filelist_len - 1;
+	    else if (selected + editwinrows * width >= filelist_len)
+		selected = (selected + editwinrows * width - filelist_len) %
+				width +	filelist_len - width;
+	    else
+		selected += editwinrows * width;
 	} else if (func == do_first_file) {
 	    selected = 0;
 	} else if (func == do_last_file) {