Cmd::Unit: changed command_name accessor to a func.¹

1: Easier than having another macro/burden on commands to add a stub
for the stupid linker stupid C++ stupid.
IO::PropStateStore: added state_of(), initialized_of(), and their negatives.

IO: doc tidy.
Cmd::Hive::Init::init_object(): corrected type masking.¹

1: D'oh. The supplied types in a PropStateStore can't really
change, so StorageInfo uses "initialized" state to say whether
a prop has storage.