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.