Better server runtime commands

Currently, server runtime control options (runtime commands) are implemented with Unix signals. This means that such commands are just one-way messages with a very limited payload. There is no way for a command sender to get the result (e.g. diag info). We must implement a new transport for more verbose communications (e.g. Unix domain sockets) for the runtime commands. The new transport should also enable runtime commands on platforms with limited signal support (e.g. OpenBSD).