Implement execin by using registered functions

[crosbymichael]

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:

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

[crosbymichael]

ping @calfonso @vishh @imain

[mrunalp]

Reviewing.

[mrunalp]

Neat! :)

[mrunalp]

+1 to reusing the exec flag.

[crosbymichael]

@mrunalp done

[mrunalp]

LGTM.

[crosbymichael]

thanks for the review