common.subr: Change handling of internal errors. They now - return 64 or 70 - print always an error message to stderr - set the given option variable to the null value - unset OPTARG in *every* case. This is independent whether the short options start with an `:' character or not.

syntax: regexp

^work-default/
^work-alldeps/
^docs/_build/
^docs/_venv.*/
^_venv.*/
^tests/fbhyve/bootvars.fd