speller: fix replacing marked text in the alternate spell checker

With read_file() revamped, it now uses partition_filestruct() indirectly
via ingraft_buffer(), so we can't use partition_filestruct() to replace
marked text in the alternate spell checker anymore without segfaulting.

Add the new function replace_marked_buffer() to accomplish this instead.
Based on replace_buffer(), it uses extract_buffer() to throw away the
marked un-spell-checked text, and then uses read_file() to insert the
spell-checked text at the position where the mark was.

Accordingly, remove unneeded partitioning and related stuff from
do_alt_speller().  Besides pasting the file into the buffer at
current[current_x], ingraft_buffer() also deals with renumbering,
updating totsize, and handling a magicline, so do_alt_speller()
doesn't need to do those anymore.

src/files.c

@@ -545,6 +545,39 @@ void replace_buffer(const char *filename)
     /* Put current at a place that is certain to exist. */
     openfile->current = openfile->fileage;
 }
+
+#ifndef NANO_TINY
+/* Open the specified file, and if that succeeds, blow away the text of
+ * the current buffer at the given coordinates and read the file
+ * contents into its place. */
+void replace_marked_buffer(const char *filename, filestruct *top, size_t top_x,
+	filestruct *bot, size_t bot_x)
+{
+    FILE *f;
+    int descriptor;
+    bool old_no_newlines = ISSET(NO_NEWLINES);
+    filestruct *trash_top = NULL;
+    filestruct *trash_bot = NULL;
+
+    descriptor = open_file(filename, FALSE, TRUE, &f);
+
+    if (descriptor < 0)
+	return;
+
+    /* Don't add a magicline when replacing text in the buffer. */
+    SET(NO_NEWLINES);
+
+    /* Throw away the text under the mark, and insert the processed file
+     * where the marked text was. */
+    extract_buffer(&trash_top, &trash_bot, top, top_x, bot, bot_x);
+    free_filestruct(trash_top);
+    read_file(f, descriptor, filename, FALSE, TRUE);
+
+    /* Restore the magicline behavior now that we're done fiddling. */
+    if (!old_no_newlines)
+	UNSET(NO_NEWLINES);
+}
+#endif /* !ENABLE_TINY */
 #endif /* !DISABLE_SPELLER */
 
 /* Update the screen to account for the current buffer. */