size_t fmt_fill(char *dest,
size_t srclen, size_t padlen,
DESCRIPTIONfmt_fill appends padlen-srclen spaces (if that number is positive) to dest (which holds srclen bytes). It truncates the output only if the length would exceed maxlen.
It returns the number of bytes it wrote.
fmt_fill does not append \0.
If dest equals FMT_LEN (i.e. is zero), fmt_fill returns the number of bytes it would have written.