Name the attach reply statuses instead of magic numbers

Add an AttachStatus enum to ps2-library.zig for AttachReply.status,
distinguishing the three failure causes handleAttach previously
collapsed into a bare -1: invalid_request (message too short),
missing_endpoint (no capability passed), and no_such_device (no port
identified the requested device type). The keyboard and mouse drivers
check against AttachStatus.ok rather than a literal 0.

system/drivers/ps2-bus/keyboard.zig

@@ -109,7 +109,7 @@ pub fn main(init: runtime.process.Init) void {
         return;
     };
     if (attached.len < @sizeOf(ps2.AttachReply) or
-        std.mem.bytesToValue(ps2.AttachReply, attach_reply[0..@sizeOf(ps2.AttachReply)]).status != 0)
+        std.mem.bytesToValue(ps2.AttachReply, attach_reply[0..@sizeOf(ps2.AttachReply)]).status != @intFromEnum(ps2.AttachStatus.ok))
     {
         _ = runtime.system.write("system/drivers/ps2-bus/keyboard: attach refused\n");
         return;