propolis-standalone should be able to start without a serial console consumer

At present when you start propolis-standalone, the VM will not begin executing until a process has connected to the ttya socket that is created in the current directory.

Waiting before booting is often critical when you want to catch the boot loader or do some early boot debugging. At other times, though, such as while running automated tests, it would be good to be able to skip the wait and just begin booting.

Things that it might be nice to be able to configure in the file related to this:

• the path to the socket, which appears hard-coded to ttya in the current directory today
• whether or not to hold boot waiting for a consumer on the socket
• if not waiting, what should we do with data that arrives while no consumer is connected? buffer and pause, drop, etc?

Start at the propolis-standalone startup path and configuration handling, then trace where creation of the ttya socket blocks VM execution. Clarify the socket path, wait behavior, and handling of output without a consumer with maintainers; done means these behaviors are configurable and verified by appropriate tests.