Support QAModeTermCode var for unattended shutdown on LL_ERRS.

Normally LL_ERRS in a developer build of the viewer pops up an OS message box
with the text of the LL_ERRS log message, requiring user intervention to
proceed. This isn't good for unattended testing, especially on Parabuild
machines. Running the viewer with '--set QAModeTermCode n' for some
0 < n < 256 will, on LL_ERRS, cause the viewer to terminate immediately with
_exit(n).