Fix a mistake in a rarely-issued error message.

* onetime
  (Configuration.register): Pass correct format version to pad.id().
Make the "Format:" plaintext header represent only format level.

This is part of the shift to using so-called "inner headers" to
specify detailed format information, as revived in commit 04974c7fe.
That shift is not complete yet, but the changes below take care of the
user-visible part of it.

* onetime
  (Format_Level): Replaces Format; redocument.
  (FormatLevel): Replaces FormatVersion exception.
  (Pad.convert): Take format_level as a string instead of
    format_version as a number.
  (PadDecoder.decode): Check format_level now, as a string.
  (PadDecoder.__init__): Take format_level instead as a string instead
    of input_format as a number.  Also, get rid of an error check that
    will no longer happen here, because it will happen where the real
    format version, the one embedded in the ciphertext, is obtained.
  (Pad._make_fuzz): Update comments accordingly.
  (Configuration._get_next_offset): Update doc string.
  (Pad._get_original_format_level_id): Replaces Pad._get_v1_id.
  (Pad.__init__, Pad.id, Configuration.register, main): Adjust for above.
Consistify some terminology, following up to commit 0d6934110.

* onetime
  (PadDecoder.__init__): Doc fix and error message fix.
  (PadDecoder.decode): Comment fix.