The new DWIM `erc-tab' command defers to pcomplete inside the prompt
area and runs a hook otherwise. But the hook's interface demands that
implementing members handle a raw prefix arg for no compelling reason.
And most of these members are movement-focused commands whose natural
interactive spec tends to be numeric, making for some roundabout
boilerplate awkwardness. This patch switches to numeric prefixes
everywhere, which is how things likely should have been originally.