exec: Apply more headroom to avoid E2BIG

I ran into "argument list too long" errors with a

    bfs -type f -exec grep pattern '{}' +

command, despite the current accounting being pretty careful.  Some
experimentation showed that an additional 2048 bytes of headroom is
always safe.

While we're at it, explicitly account for the terminating NULL pointers
in argv and environ.

0 files changed, 0 insertions, 0 deletions