Write an octal ascii representation of an unsigned long integer
#include <fmt.h>
size_t fmt_8long(char *dest,size_t source);
fmt_8long writes an ASCII representation ('0' to '7', base 8) of source to dest and returns the number of bytes written.
fmt_8long does not append \0.
If dest equals FMT_LEN (i.e. is zero), fmt_8long returns the number of bytes it would have written.
For convenience, fmt.h defines the integer FMT_8LONG to be big enough to contain every possible fmt_8long output plus \0.