Before truss would fetch 100 string pointers and happily walk off the end
of the array if it never found a NULL. This also means for a short argv
list it could fail entirely if the 100 string pointers spanned into an
unmapped page.
Instead, fetch page-aligned blocks of string pointers in a loop fetching
each string until a NULL is found.
While here, make use of the open memstream file descriptor instead of
allocating a temporary array. This allows us to fetch each string once
instead of twice.
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