* lib/full-read.h, lib/full-write.h, lib/safe-read.h, lib/safe-write.h:
Include idx.h.
* lib/full-write.c (full_read, full_write):
Now accept and returns idx_t.
* lib/safe-read.c (bufptr): New type, since apps are not
supposed to #define keywords like ‘const’.
(safe_read, safe_write): Now accept idx_t and return ptrdiff_t.
* lib/safe-read.h (SAFE_READ_ERROR):
* lib/safe-write.h (SAFE_WRITE_ERROR):
Now ptrdiff_t, not size_t.
* modules/full-read, modules/full-write, modules/safe-read:
* modules/safe-write (Depends-on): Add idx.