Rework the argv and env string fetching for execve to be more robust.

Rework the argv and env string fetching for execve to be more robust.
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.