xsnap should exit quietly upon command pipe EOF

[warner]

What is the Problem Being Solved?

@mhofman and I were examining some logs of the load generator, and noticed that when the kernel is shut down (SIGTERM to the kernel process), stderr is immediately filled with a bunch of error messages from the xsnap worker processes, with lines like:

Cannot read netstring, reading length prefix, fscanf

I think this is the netstring reader in the worker, which gets an end-of-file indication rather than a netstring length prefix header.

I'm thinking that the worker process should silently exit when it sees this happen, instead of complaining. Having the parent process exit is a perfectly reasonable time for the worker process to exit: it's the least exceptional form of shutdown we've got.