usb: plug and play -- hot-plug, unplug, every interface, real mice

Devices can now arrive and leave after boot. A "usbhotplug" kernel thread
handles root-port changes, which it is told about by Port Status Change
events and also checks for every quarter second. It also handles changes
behind hubs, which each hub reports on its status-change endpoint. An
unplugged device has its endpoints closed and any transfer in flight
failed. Then its drivers' disconnect hooks run, the slot is disabled and
its memory is freed. Everything behind a hub that goes goes with it. This
needed the interrupt handler to file command and endpoint-0 completions
for their waiters, so that enumeration can run while keyboards interrupt.

On real hardware: the controller is taken from the firmware through USB
Legacy Support before reset, with SMIs off. On Intel 7/8/9 series parts
the ports are switched over from EHCI, which is where firmware leaves
them.

Every interface of a device is offered to the class drivers, not just
the first, so a receiver's mouse on interface 1 gets a driver. Configure
Endpoint now copies the slot context from the controller. Before, it
rebuilt it and lost the route string and TT fields of devices behind
hubs.

HID: pointers are read in report protocol from their report descriptor.
That covers buttons, X/Y and wheel, relative or absolute, and report IDs.
Boot protocol is kept as the fallback for mice and for keyboards.
Non-pointer HID interfaces are declined rather than treated as tablets.
USB keyboards get software key repeat. Mass storage handles up to four
sticks, each unregistered from the block layer when it is pulled out,
along with its cached blocks.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HH7xukt4aaiGj1nQEAoQf1

kernel/core/main.c

@@ -10,6 +10,7 @@
 #include "irq.h"
 #include "dma.h"
 #include "klog.h"
+#include "usb.h"
 
 static void *klog_base;   // the persistent log, just above the kernel image
 
@@ -365,6 +366,9 @@ kmain(uint32 fb_addr)
   // For the same reason: a kernel thread. See irq_poll_thread() for the
   // lines it is there for, which on real hardware is most PCI INTx.
   irq_poll_start();
+  // After the poller, which is what keeps a USB controller whose INTx
+  // never arrives working, and after the scheduler, which it sleeps in.
+  usb_hotplug_start();
   // The debugger's own thread, for the same reason netinit's comes after
   // sinit: kthread_create needs a usable process table. From here on Ctrl-G
   // on the console answers, with no filesystem mounted and nothing in