Route all returns from the interrupts and faults through the doreti_iret

labeled iretq instruction.

Suppose that multithreaded process executes two threads, currently
scheduled on different processors. Let assume that thread A executes
using %cs or %ss pointing into the descriptor from LDT. If IPI comes
which handler does not return by jump to doreti, and meantime thread B
invalidates descriptor pointed to by %cs or %ss, then iretq from IPI
handler could fault.

Routing the return by doreti_iret allows kernel to catch the situation
and recover from it by sending signal to the usermode.

Tested by:	pho
MFC after:	1 week

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