Various fixes to the sidebar logic.

Various fixes to the sidebar logic.

Use strfcpy instead of strncpy.  The current logic could write past
the end of the buffer.

Don't mess with BUFFY next pointers during removal.  The
mutt_parse_mailboxes() is fine, but this is still not something that
should be done inside sidebar.c.

On removal, set next->prev since we can.

Fix unmailboxes logic:
  * only fix the prev pointers once.
  * if we unmailbox the open mailbox, set it to NULL.

Lastly, flag a redraw on mailboxes/unmailboxes.