Fix bug in OLDGNU format creation.

Fix bug in OLDGNU format creation.

See tests/append02.at for a detailed description

* src/common.h (MODE_FROM_HEADER): Take additional argument.
(mode_from_header): Likewise.
* src/create.c (mode_to_chars): Store all mode bits if
using OLDGNU_FORMAT. This reverses f4e4adea80a.
* src/list.c (decode_header): Use header mode field
to discern between GNU and OLDGNU formats.
(mode_from_header): Store unrecognized mode bits (from 10th up)
in the location pointed to by the third parameter.
* tests/append02.at: Update documentation and references.