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/Process.cpp

@@ -705,6 +705,8 @@ int Process::do_exec(String path, Vector<String> arguments, Vector<String> envir
     new_main_thread->make_thread_specific_region({});
 
     memset(&tss, 0, sizeof(TSS32));
+    tss.iomapbase = sizeof(TSS32);
+
     tss.eflags = 0x0202;
     tss.eip = entry_eip;
     tss.cs = 0x1b;