exec: Find ARG_MAX with binary search after E2BIG

Previously we would shrink the command by one argument at a time until a
successful execution.  This is okay if the ARG_MAX estimate is just a
little bit off, but is terribly slow when it's off by a lot.

One situation where it's very far off is when a 32-bit build of bfs
launches a 64-bit binary.  In this case, bfs thinks the argv pointers
are 4 bytes, while they actually take up 8 bytes.  The performance is
quite bad:

    $ time ./bfs-64 ~/code/linux -exec echo {} + >/dev/null
    ./bfs-64 ~/code/linux -exec echo {} + > /dev/null  0.03s user 0.07s system 99% cpu 0.095 total
    $ time ./bfs-32 ~/code/linux -exec echo {} + >/dev/null
    ./bfs-32 ~/code/linux -exec echo {} + > /dev/null  0.08s user 10.33s system 100% cpu 10.390 total

After this change, performance is much better:

    $ time ./bfs-32 ~/code/linux -exec echo {} + >/dev/null
    ./bfs-32 ~/code/linux -exec echo {} + > /dev/null  0.03s user 0.08s system 99% cpu 0.110 total

0 files changed, 0 insertions, 0 deletions