flatten ExecOpts into container.exec params

[vito]

fixes #3340

I ended up needing this because I need to be able to pass Stdin but that results in setting RedirectStdout and RedirectStderr to an empty string because the ExecOpts struct wasn't a zero-value. Without this change I would get this from the shim:

15: [0.11s] panic: open : no such file or directory
15: [0.11s]
15: [0.11s] goroutine 1 [running]:
15: [0.11s] main.run()
15: [0.11s] 	/src/main.go:47 +0xad9
15: [0.11s] main.main()
15: [0.11s] 	/src/main.go:91 +0x19

So this promotes these params to exec params, removing the nesting, and removes the "" vs. nil distinction for stdin, redirectStdout, and redirectStderr.

I don't think there was any difference between stdin: "" and stdin: nil before anyway; one would set it to an empty file, and the other would send it to /dev/null. From the process's perspective both would just yield EOF.

I decided to keep a ContainerExecOpts type around under core since it feels nicer than passing string, string, string. 😄