Simplify task switches

No longer using the rsp from the entry to the kernel, but instead
switching rsp at task-switching time in assembly.

This currently breaks fork()

src/kernel/log_areas.inc

@@ -1,8 +1,8 @@
 LOG(apic,   info);
-LOG(device, debug);
-LOG(paging, info);
-LOG(driver, debug);
+LOG(device, info);
+LOG(paging, debug);
+LOG(driver, info);
 LOG(memory, debug);
-LOG(fs,     debug);
+LOG(fs,     info);
 LOG(task,   debug);
 LOG(boot,   debug);