init: Introduce switchroot command

This sets PATH, LD_LIBRARY_PATH and runs init scripts for the new root.
In the future this could replace the entire set of services that should
run, restarting any services for which a newer executable is available
in the new root and stopping any services which don't exist in the new
root. This behavior could also be useful for soft-reboots in the future
to handle updates without rebooting the entire system.

bootstrap/src/exec.rs

@@ -100,6 +100,7 @@ pub fn main() -> ! {
             .filter(|var| !var.starts_with(b"INITFS_"))
             .collect::<Vec<_>>()
     };
+    envs.push(b"RUST_BACKTRACE=1");
     //envs.push(b"LD_DEBUG=all");
     envs.push(b"LD_LIBRARY_PATH=/scheme/initfs/lib");