derive_probes(): properly handle optionality

derive_probes(): properly handle optionality

We previously overrode a location's optionality if it was derived from a
globby process probe. However, this causes further probe points to be
derived to also be optional, even though the 'higher level' probe point
had its optionality restored to what the user really wants. The end
result was that we would list probes as not optional even though the
endpoint probe was optional.

What we really mean when we say process probes born out of a higher
globby process probe should be optional is that only some of the matched
binaries will actually resolve, so it's OK if some don't resolve.
However, those that *do* resolve should respect the user's wish with
regards to optionality.

To do this, we must not override the probe_point's optional flag in
derive_probes.

This is mostly a cosmetic change, although some runtime elements do rely
on optionality to decide whether to emit a warning upon failing to
register a probe.

This patch also makes process.library probes born out of a higher globby
library probe follow the same logic.