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

1 files changed, 2 insertions, 0 deletions

diff --git a/exec.h b/exec.h
index 1ba409f..a3e3c71 100644
--- a/exec.h
+++ b/exec.h
@@ -63,6 +63,8 @@ struct bfs_exec {
 	size_t arg_size;
 	/** Maximum arg_size before E2BIG. */
 	size_t arg_max;
+	/** Lower bound for arg_max. */
+	size_t arg_min;
 
 	/** A file descriptor for the working directory, for BFS_EXEC_CHDIR. */
 	int wd_fd;