Split the function of the PCB_FPU flags into two: PCB_FPU now indicates that
the actual FPU is enabled, while PCB_FPREGS indicates that the FPU state
structure in the PCB is valid. This separation reflects the situation on
FPU-less systems in which the FP state is used by the emulator but we don't
actually want to try to turn on the non-existant FPU.

Use this flag to save and restore FP regs properly on both AIM and Book-E.
As a side effect, this sets up hard-FP and Altivec on Book-E CPUs with such
abilities except for a trap handler to call enable_fpu()/enable_altivec().
Deregister helper hooks on vnet destroy.
Unify handling of illegal instruction faults between AIM and Book-E. This
allows FPU emulation on AIM as well as providing support for the mfpvr
and lwsync instructions from userland on e500 cores. lwsync, in particular,
is required for many C++ programs to work correctly.

MFC after:	1 week
In r257692 I intentionally deleted code that handled P2P interfaces
with equal addresses on both sides. It appeared that OpenVPN uses
such configutations.

Submitted by:	trociny