SL-10153: Review and rationalize fetching paths from environment.

Use LLStringUtil::getenv() or getoptenv() whenever we fetch a string that will
be used as a pathname.

Use LLFile::tmpdir() instead of getenv("TEMP").

As an added extra-special bonus, finally clean up $TMP/llcontrol-test-zzzzzz
directories that have been accumulating every time we run a local build!