Let Daemon.spawn stick random stuff into argv

Let Daemon.spawn stick random stuff into argv

Summary:
`Daemon.spawn` starts a new process. That new process calls
`Daemon.check_entry_point` and never returns. This always happens before
command line args are parsed.

That means you can really put whatever you like into the arg list for
the spawned process.

The Flow team often gets questions about why are there so many flow
processes. These questions are usually from people who are looking at
`ps`. Well, we can make these processes self-documenting but sticking
reasons directly into their args.

Unfortunately, this doesn't help processes which are forked. They'll
always have the same args as their parent process.

Reviewed By: alexchow

Differential Revision: D6264263

fbshipit-source-id: f3fd313b9a4a1db885b8e146ecc2836129fdb2a1