Major cleanup of akaros fork/exec code

We now longer have any fork/exec calls at all.  We support StartProcess
directly.  We do support a direct Exec calls, but the implementation for
Exec was wrong and I fixed it as part of this commit.

One nice observation is taht we don't need the ForkLock in Akaros
because we aren't forking and don't have races with file descriptors
magically appearing / disappearing out from under us while executing a
fork/exec command.