Prefer off_t to uintmax_t for continued_file_*

Prefer off_t to uintmax_t for continued_file_*

* src/buffer.c (continued_file_size, continued_file_offset):
Now off_t, not uintmax_t.  All uses changed.
* src/common.h (UINTMAX_FROM_HEADER):
* src/list.c (uintmax_from_header):
Remove; unused.
* src/list.c (simple_print_header):
* src/xheader.c (volume_size_decoder, volume_offset_decoder):
Treat offset as off_t, not uintmax_t.