Too small argv limit

Current kernels don't have a fixed argv/environ limit any more, but the user-space emulation of qemu is still using a fixed limit.  This can cause execve to fail when it wouldn't on a real system.  For example, the follwing command should not fail in the emulated environment:

$ /bin/true $(yes | head -n 100000)
-bash: /bin/true: Argument list too long

This was fixed in QEMU 2.5.