remove mounts automatically

Also combined all these little mount and cleanup helpers into one
`cleanup` script with flags.

debian/create_environment

@@ -20,7 +20,7 @@ fi
 if test -e "$ROOT"
 then
     echo "Existing installation found at $ROOT, removing."
-    ./destroy_environment
+    ./cleanup
 fi
 
 echo "Copying $FRESH to $ROOT."
@@ -28,7 +28,7 @@ cp -r "$FRESH" "$ROOT"
 
 echo "Initializing $ROOT."
 
-./mountpoints
+./cleanup add_mounts
 
 # Don't bother creating a new tmpfs. We don't want to leak files in, and we
 # don't want to waste more RAM on a second tmpfs. The whole thing is
@@ -38,3 +38,5 @@ chmod 1777 "$ROOT/tmp"
 cp -r install_scripts "$ROOT/root"
 
 chroot "$ROOT" /root/install_scripts/user_setup "$@"
+
+./cleanup mounts