fprintftime: sbyte_count_t to fix some overflows

fprintftime: sbyte_count_t to fix some overflows

* lib/strftime.c: Include <stdint.h>, for PTRDIFF_MAX.
(byte_count_t): Now a typedef instead of a macro.
(sbyte_count_t): New typedef.
(SBYTE_COUNT_MAX): New macro.
(memset_byte, width_add, __strftime_internal):
(fwrite_lowcase, fwrite_uppcase) [FPRINTFTIME]:
Avoid overflow issues with fprintftime by using byte_count_t
or sbyte_count_t instead of size_t or int.
* modules/nstrftime: Depend on stdint-h.