When piping a text attachment, there is no reliable way to know the
charset used.

Vincent Lefèvre says:
  It was decided in the past that when there is no information on the
  charset in a transmission to an external command (e.g. as for mail
  composing), texts are expected to be transmitted in the local
  charset.

Add a MUTT_CHARSET flag to enable charset conversion on text
attachments for both when $attach_split is set and unset.