Revamp Passenger Standalone runtime installation, and make agent installation easier.

Passenger Standalone no longer has its own runtime that's separate from the
Nginx and Apache version. Everything now looks for the agent and for the Nginx
engine in two locations: the support binaries dir and the home dir. This significantly
simplies runtime lookup logic.

The Passenger Standalone runtime installer has now been split into multiple
smaller `passenger-config` subcommands:

  download-agent
  compile-agent
  install-agent (combines download and compile)
  download-nginx-engine
  compile-nginx-engine
  install-nginx-engine (combines download and compile)
  install-standalone-runtime

This also makes it easier for future commands to depend on the agent.