[Toybox] xargs

[enh]

okay, finally found time to work out why the new xargs tests fail on
Android. (as you can probably guess from the patch to differentiate
the error messages again!)

it turns out that the problem is to do with the old argument about
"what's the kernel limit, really?". remember that thread where Linus
said "I suspect a 128kB sysconf(_SC_ARG_MAX) is the sanest bet"?
(https://lkml.org/lkml/2017/11/1/946.)

bionic made that change, which means it can't pass your two new tests
(because they require a larger limit than that).

glibc still uses max(rlimit(stack)/4, ARG_MAX) like we used to, so they pass.

musl seems to just use ARG_MAX like bionic, so i assume the two new
tests are broken on musl too?

i can easily flip bionic back, but since you were the person most in
favor of this change in the first place, i'm assuming you'd rather
change the tests? (though i'm not sure what _to_. arithmetic on
`$(getconf ARG_MAX)`?)