Unsafe mode for probes

[Frank Ch. Eigler]

Hi -

> "a proper context structure allocation for temporary values" is
> one of the things I would like to wrap into a stable API, and
> do not enforce it.

But a context is not an optional safety check sort of thing.  It is
the place where all the runtime per-probe-execution data is, including
error flags, temporaries, self-monitoring counters, printing buffers,
all kinds of stuff.  We can probably make it smaller for probe
handlers that are trivially simple.  But code just won't compile/run
with no context.

> "Unsafe" means that the stack safety is up to the author.
> For example, if I hook only entry points of the system calls I
> want the freedom to inject custom C code.

Maybe "raw" would be a better name then: a probe handler that
is bound to use self-contained embedded-C only (thus can't print,
deal with stap globals, etc.).  Even providing $context vars
would be tricky.

- FChE