Undo the "quoted printable" encoding in buffer from BEG to END.
If the optional argument WRAPPER is non-nil,
we expect to find and remove the wrapper characters =?ISO-8859-1?Q?....?=.
On encountering malformed quoted-printable text, exits with an error,
unless NOERROR is non-nil, in which case it continues, and returns nil
when finished. Returns non-nil on successful completion.
If UNIBYTE is non-nil, insert converted characters as unibyte.
That is useful if you are going to character code decoding afterward,
as Rmail does.
(fn BEG END &optional WRAPPER NOERROR UNIBYTE)