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;

system/drivers/ps2-bus/mouse.zig

@@ -82,7 +82,7 @@ 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/mouse: attach refused\n");
         return;

system/drivers/ps2-bus/ps2-bus.zig

@@ -67,25 +67,25 @@ var port_device_types = [_]?ps2.DeviceType{ null, null };
 /// Writes an `AttachReply` into `out` and returns its length.
 fn handleAttach(message: []const u8, got: ipc.Received, out: []u8) usize {
     const reply = struct {
-        fn write(buffer: []u8, status: i32) usize {
-            const header = ps2.AttachReply{ .status = status };
+        fn write(buffer: []u8, status: ps2.AttachStatus) usize {
+            const header = ps2.AttachReply{ .status = @intFromEnum(status) };
             @memcpy(buffer[0..@sizeOf(ps2.AttachReply)], std.mem.asBytes(&header));
             return @sizeOf(ps2.AttachReply);
         }
     };
 
-    if (message.len < @sizeOf(ps2.AttachRequest)) return reply.write(out, -1);
+    if (message.len < @sizeOf(ps2.AttachRequest)) return reply.write(out, .invalid_request);
     const request = std.mem.bytesToValue(ps2.AttachRequest, message[0..@sizeOf(ps2.AttachRequest)]);
-    const endpoint = got.cap orelse return reply.write(out, -1); // no endpoint passed
+    const endpoint = got.cap orelse return reply.write(out, .missing_endpoint);
 
     for (&port_device_types, 0..) |maybe_type, port_index| {
         const device_type = maybe_type orelse continue;
         if (@intFromEnum(device_type) != request.device_type) continue;
         port_endpoints[port_index] = endpoint;
         writeLine("system/drivers/ps2-bus: {s} driver attached\n", .{@tagName(device_type)});
-        return reply.write(out, 0);
+        return reply.write(out, .ok);
     }
-    return reply.write(out, -1); // no port identified as that device type
+    return reply.write(out, .no_such_device);
 }
 
 pub fn main() void {

system/drivers/ps2-bus/ps2-library.zig

@@ -285,7 +285,18 @@ pub const AttachRequest = extern struct {
     device_type: u32,
 };
 
-/// Reply to an `AttachRequest`. `status` is 0 on success or a negative errno.
+/// How the bus answered an `AttachRequest` (`AttachReply.status`).
+pub const AttachStatus = enum(i32) {
+    ok = 0,
+    /// The request was malformed (too short to be an `AttachRequest`).
+    invalid_request = -1,
+    /// The call carried no endpoint capability to forward to.
+    missing_endpoint = -2,
+    /// No port identified a device of the requested type.
+    no_such_device = -3,
+};
+
+/// Reply to an `AttachRequest`. `status` is an `AttachStatus` value.
 pub const AttachReply = extern struct {
     status: i32,
     _padding: u32 = 0,