Revisit and fix the "dlopen for DSO" issue with symbols LLVM can't find.

It was reported that although we took care of (almost all) function
lookups, that wasn't good enough because non-function symbols could also
be a problem. In particular, there were several ustrings referenced
externally.

The solution is just to declare them static in the functions where they
are used. I verified that it creates those static ustrings just once for
each shader, not on every shader execution. Also found a couple stray
functions I didn't catch before.

I also modified the testsuite runtest.py, so that it checks an
environment variable OSL_TESTSHADE_NAME, and if set, uses that to
override "testshade". This lets me run the full testsuite with
testshade_dso rather than testshade, verifying that we now have things
working. (Well, as best as can be verified by the coverage afforded by
the testsuite.)
Gracefully handle the USE_MCJIT=0 case for old LLVM without MCJIT libs
Allow per-shader-group outputs (which will not be optimized away).

Previously, the "string[] renderer_outputs" attribute was a global (for
the whole shading system) list of render output names (aka "AOVs"),
having the primary effect that they would not be elided during shader
optimization, even if they were not explicitly connected to downstream
shaders.

This patch adds (additionally) a per-shader-group attribute of the same
name and purpose, so you could designate group-specific output names.
Every place where the optimizer previously checked the global list, it
now also checks the list for the specific group it is optimizing.

This is really useful for apps that use OSL as a computation engine or
pattern language, where the primary output is not Ci or other standard
OSL "global variables," and where furthermore you may want a different
set of named outputs for each group, so no global (ShadingSystem-wide)
list will suffice.

Apps or renderers that do not have this problem may continue just using
the old attribute, and need not set the renderer outputs for each group
individually.