Mk/Scripts/do-fetch.sh: read distfiles from stdin to avoid ARG_MAX

[bapt]

Summary

Pass the list of files to fetch to do-fetch.sh through stdin instead of
command-line arguments.

PR: 295029

[mat]

I'm not sure this would fix the command line length, it it just moves the length around.
I've been thinking of a real fix for this, and I think it needs to put it all in a temporary file, kinda

.   for d in ${DISTFILES}
     printf "%s\n" "$d" >> temp
.   endfor

and then pass maybe either pass the tempfile name to do-fetch, or use it as its stdin.

Also, there is the same problem for each script that we pass the distfiles to, like makesum.

[mat]

Maybe using \0 instead of \n is better there, and juggling with IFS to split on \0, in case some distfile has a strange file name.

[bapt]

yesh I pondered the tempfile, but this was longer, actually, but yes I think you are right, as printf will complain also about the ARG_MAX.