services/agent/v23agentd: break into subcommands
services/agent/v23agentd: break into subcommands

'stop' becomes its own command

The credentials are now specified as a flag (instead of positional
argument), and default to V23_CREDENTIALS.

MultiPart: 2/2
Change-Id: I4f71ee69e05f293118c095d7acc94d53ec794a05

vanadium/website,vanadium/website,vanadium/website