This is based on the trash folder patch by Cedric Duval.
Modifications to the original patch are:

* Use a flag called M_PURGE instead of M_APPENDED.  The same flag is
  then used in the following "purge" patch instead of adding a
  different flag.

* Removed the counter in context.  The existing context->deleted
  is all that's needed.

* Removed the "auto unset M_PURGE" when M_DELETED is unset inside
  _mutt_set_flag(), although this is convenient, it easily leads to
  header->purge not being reset in a few situations.

* Reset purge flag along with the deleted flag if $delete is answered
  no.

* Set M_PURGE on an edited message. (edit_one_message())

* Preserve purge flag in mutt_reopen_mailbox()

* Turn off OPTCONFIRMAPPEND when saving to the trash, rather than
  hardcoding it off in mutt_save_confirm().  That way, normal save to the
  folder will respect the option.