Pause syscall and int 0xee interrupt syscalls

The syscall/sysret instructions don't swap stacks. This was bad but
passable until syscalls caused the scheduler to run, and scheduling a
task that paused due to interrupt.

Adding a new (hopefully temporary) syscall interrupt `int 0xee` to allow
me to test syscalls without stack issues before I tackle the
syscall/sysret issue.

Also implemented a basic `pause` syscall that causes the calling process
to become unready. Because nothing can wake a process yet, it never
returns.

src/kernel/interrupts.h

@@ -9,9 +9,11 @@ enum class isr : uint8_t
 {
 #define ISR(i, name)     name = i,
 #define EISR(i, name)    name = i,
+#define UISR(i, name)    name = i,
 #define IRQ(i, q, name)  name = i,
 #include "interrupt_isrs.inc"
 #undef IRQ
+#undef UISR
 #undef EISR
 #undef ISR