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/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,