@a, @p or @r in player and console-sent commands

CMDBOOK-2312 - Reported by killjoy1221

This works in vanilla, but bukkit has yet to implement it. You could do "/give @a fish" to give everyone some fish. You could also do "/slay @r" to kill a random player.

Currently, this is impossible without using commandblocks.

Start by locating the command parsing and execution entry points for player- and console-sent commands, then trace how command arguments are resolved. Compare the current behavior with the requested @a, @p, and @r selectors; done means those selectors work for the relevant commands without requiring command blocks.

Written by the indexing model from the issue text.