Add a generic property API

There is data that libei and the EIS implementation will want to exchange that is not covered by the immediate API.

To avoid having to add APIs for all of these, let's provide a generic property API that both server and client can use to exchange this info.

The property API provides read/write/delete permissions but those only apply to the client, not the server. The idea is that a server can create (or restrict) properties that the client can read but not modify and/or delete. A special-case are properties filled in automatically by libei: ei.application.pid and ei.application.cmdline. These could be used by e.g. the portal implementation to match permissions.