shellcheck: run_trunnel.sh issues

Shellcheck (​​​​https://github.com/koalaman/shellcheck) finds the following issues:

In run_trunnel.sh line 12:
for file in `find ./src/trunnel/ -name '*.trunnel'`; do
            ^-- SC2044: For loops over find output are fragile. Use find -exec or a while read loop.
            ^-- SC2006: Use $(..) instead of legacy `..`.


In run_trunnel.sh line 13:
  python -m trunnel ${OPTIONS} $file
                               ^-- SC2086: Double quote to prevent globbing and word splitting.