add option parsing library for clients and frontends

We should have an option parsing library to make it easy for clients to have runtime options. Ideally it should also usable for frontends with the same option spec (xref tools like Dr. Memory with overlapping option owners).

Ideally it would be in C for use in C clients + frontends but harder to imagine it as a library there: the included x.h approach there is hard to librarify. Maybe could have provided headers and .c file. Or we can go with a slightly cleaner-looking C++ approach and leave C clients to fend for themselves.