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. :^)

Base/usr/share/man/man1/crash.md

@@ -33,6 +33,7 @@ kinds of crashes.
 * `-y`: Write to recently freed memory. (Tests an opportunistic malloc guard.)
 * `-X`: Attempt to execute non-executable memory. (Not mapped with PROT\_EXEC.)
 * `-U`: Attempt to trigger an x86 User Mode Instruction Prevention fault.
+* `-I`: Use an x86 I/O instruction in userspace.
 
 ## Examples