More flexible ExecBrocker

[nezetic]

ExecBrocker allows any users to remove code ranges from the scope of QBDI, letting the program execute freely when jumping outside of it.

It currently only implement one strategy to get the execution back, which works well for generic cases (classic external call to a library). But when it gives up, there is currently no way to fix its behavior.

A new user API should be added, in order to provide ways to customize how QBDI is getting control back.