usermanager: More consistent method behaviour, errors, and (pre-)events This commit refactors most of the exported usermanager methods substantially to ensure that all of them correctly check for the presence of the host and method they are calling and return consistent errors if either is incorrect or missing. It also unifies the event and logging code, so as a result there are more events being fired than before this commit (it also adds pre- events so that modules can block an action from going ahead). We don't have good test coverage of this API, but some manual testing suggests it seems to be working okay...

local data = {}
local getinfo = debug.getinfo;
local function linehook(ev, li)
	local S = getinfo(2, "S");
	if S and S.source and S.source:match"^@" then
		local file = S.source:sub(2);
		local lines = data[file];
		if not lines then
			lines = {};
			data[file] = lines;
			for line in io.lines(file) do
				lines[#lines+1] = line;
			end
		end
		io.stderr:write(ev, " ", file, " ", li, " ", lines[li], "\n");
	end
end
debug.sethook(linehook, "l");