physical nodes during swapout/swapin).

Also work around small race condition when starting up;
the acl file is created fter daemonization, and there is a
small window where the file exists but is not complete.
Use tmp file and rename to avoid the race.

It has been dead for a long time and I am tired of seeing the message.

shellinabox as the backend.

so that we can put node consoles into a browser iframe.

via shellinabox.

Also fix for xl toolstack error; teardown called with no XENBUS path,
so no way to find the bridge name. But we know what it is, so add it
to the env so that vif-bridge does not croak.

We were using pygrub to suck the kernel and ramdisk out before
booting the guest, but doing that makes it impossible for for
the guest to change its kernel and reboot.

Other minor fixes.

1. The backend process that starts the XEN vm (xl/xm) was not happy
   with the signal mask settings. By pass that for the vnodeboot call
   so that xl/xm sees a clean mask. The sympton was processes getting
   left behind.

2. Watch for a XEN container being rebooted from inside, by waiting a
   little bit to see if comes back on its own. We used to fold right
   away, but now keep running.

As of FreeBSD 9, the race condition in mountd has been addressed.
We automatically (based on the OS running) add the option to mountd (-S)
as part of ops-install. There is also a configuration variable, NFSRACY,
that we auto-detect and set in elabinelab setup. For regular installs,
you need to set this manually in the defs-* file. Note that the setting
in the defs file does not affect very much. It is passed to the client-side
which uses it in one copy function (used as a fall back for getting the
topology file).

There are still a couple of places in the Emulab code where handling
the race condition is hardwired into the code (most notably, imagezip).
These remain.

The -n option says to turn off the log file. We do not want to store
thes on the XEN host, will take too much room.

The -C option adds a circular buffer to store the last 8K of output
from the console. When a new client connects, dump the contents of
the circ buffer so the user sees context. At this point no longer
save output. When the user disconnects, start saving the output again
in the buffer, to dump at the next connection.

trying to start the testbed.

info from the capture process we now start for each VM. Capture
cannot do this directly, since experimental nodes cannot talk
to the capserver.

We need lock/unlock to atomically check quotas.
The explicit IsUnlimited, SetUnlimited methods are to avoid a potentially
ambiguous convention (is no quota, unlimited? is a zero quota unlimited?)