`program_invocation_name` is defined by the system headers only on
Linux, or at least it is not available on FreeBSD.
Introduce own global variable that contains the program name. If
it is unset then use `getprogname()` or `program_invocation_name`.
Co-authored-by: Ken Sedgwick <ken@bonsai.com>
CLBOSS did not know the `--developer` flag and generated a usage
message and exited. Fixed with:
- recognize and consume the `developer` flag, no actions for now
- print unrecognized options to std::cerr
Fixes ([#185])
