maint: avoid int64_t and similar types unless they're needed

maint: avoid int64_t and similar types unless they're needed

C11 doesn't require them, even POSIX doesn't strictly require the
64-bit versions, and it makes the code a bit clearer if they're
used only when needed.
* src/copy.c (write_zeros, extent_copy):
* src/extent-scan.h (struct extent_info.ext_length):
Use off_t, not uint64_t, for a value derived from a file offset.
* src/extent-scan.h (struct extent_info.ext_flags)
Prefer plain unsigned int to uint32_t where either will do.
(struct extent_scan.ei_count):
Use size_t, not uint32_t, for a value bounded by SIZE_MAX.
* src/factor.c (MAGIC64, MAGIC63, MAGIC65):
Remove unnecessary casts to uint64_t.