Handle different iret behavior in user vs kernel space

iret has different behavior in protected mode than in user-mode, as noted at https://www.felixcloutier.com/x86/iret:iretd:iretq notes that. In fact, there are multiple kinds of returns, that can be specified by NT and VM flags.

This issue is to account for the multiple behaviors in DynamoRIO where needed, like in the decoder IR.