Support multiple access levels for runtimes

[jonnor]

To allow people to use runtimes around them, we want to support multiple levels of access for non-authorized users. In escalating order of privilege:

• Read exported (toplevel) ports (capability protocol:runtime:readonly could be added for read-only)
• Read&write exported ports (capability protocol:runtime provides sending packets)
• See graph (including edge introspection, currently component:getsource but see component:getsource is used to get main graph in live mode #390)
• Change graph (but not persist, restarts will reset back, capability protocol:graph, requires ability to see graph as well)
• Persist graph changes (capability network:persist which is not supported by any runtime or client yet)

People who have the secret (passphrase) for the runtime, will always have full rights, including ability to change non-auth access level and secret.