Implement execin by using registered functions

This also changes the functionality of the default exec in to just be an
existing func that is called than handles the implementation to exec a
user user's process inside the container.  This implements this
functionallity in nsinit but is a base for how we will be handling these
types of features inside docker.

Output should look like:

```bash
0ff591c518ad241ed1d3e9a7c587505c0d6faa569d67b9301fe571d81683828b|⇒ nsinit exec --list
NAME      USAGE
exec      execute a process inside an existing container
mknod     mknod a device inside an existing container
ip        display the container's network interfaces
0ff591c518ad241ed1d3e9a7c587505c0d6faa569d67b9301fe571d81683828b|⇒ nsinit exec --func ip
NAME      MTU       MAC       FLAG          ADDRS
lo        65536               up|loopback   127.0.0.1/8,::1/128
0ff591c518ad241ed1d3e9a7c587505c0d6faa569d67b9301fe571d81683828b|⇒ nsinit exec sh
/ $ ps aux
PID   USER     COMMAND
    1 daemon   sh
   11 daemon   sh
   13 daemon   ps aux
/ $
```
Closes #7