Kernel: Prevent executing I/O instructions in userspace

All threads were running with iomapbase=0 in their TSS, which the CPU
interprets as "there's an I/O permission bitmap starting at offset 0
into my TSS".

Because of that, any bits that were 1 inside the TSS would allow the
thread to execute I/O instructions on the port with that bit index.

Fix this by always setting the iomapbase to sizeof(TSS32), and also
setting the TSS descriptor's limit to sizeof(TSS32), effectively making
the I/O permissions bitmap zero-length.

This should make it no longer possible to do I/O from userspace. :^)

Kernel/Thread.cpp

@@ -59,6 +59,7 @@ Thread::Thread(Process& process)
     m_fpu_state = (FPUState*)kmalloc_aligned(sizeof(FPUState), 16);
     memcpy(m_fpu_state, &s_clean_fpu_state, sizeof(FPUState));
     memset(&m_tss, 0, sizeof(m_tss));
+    m_tss.iomapbase = sizeof(TSS32);
 
     // Only IF is set when a process boots.
     m_tss.eflags = 0x0202;