This module, except for useEnv, is copied from the build-env package.
fchroot runs an IO action inside a chroot fchroot performs a chroot, runs the action, and then restores the original root and working directory. This probably affects the chroot and working directory of all the threads in the process, so... NOTE: will throw IOError if internal chroot fails
The ssh inside of the chroot needs to be able to talk to the running ssh-agent. Therefore we mount --bind the ssh agent socket dir inside the chroot (and umount it when we exit the chroot.
A function to force the process output by examining it but not printing anything.