ipc: rename COMMAND to RUN_COMMAND for consistency (#2956)

ipc: rename COMMAND to RUN_COMMAND for consistency (#2956)

All other message types are verbs, only our first-ever message COMMAND wasn’t.

While we’re here, also change the message type dictionary into a table with
clickable links to the corresponding reply type.

Authors of downstream IPC libraries are encouraged to keep the old name around
so as to not break existing code, but mark it as deprecated.