[Offload] Move (most) global state to an OffloadContext struct

[RossBrunton]

Rather than having a number of static local variables, we now use
a single OffloadContext struct to store global state. This is
initialised by olInit, but is never deleted (de-initialization of
Offload isn't yet implemented).

The error reporting mechanism has not been moved to the struct, since
that's going to cause issues with teardown (error messages must outlive
liboffload).

[callumfare]

The global state stuff looks good to me, but I wonder if we can make tracing of olInit work. Rather than disable tracing for olInit, we could special-case it so it emits both parts of the trace after the actual function call, rather than before and after. If we did that I think it's ok to have a special check for the function name being olInit rather than adding a new property to the function record type.

[RossBrunton]

@callumfare I've just updated it so that isTracingEnabled just checks to see whether offload has been inited, and if it hasn't just assumes false.