Meson: Refactor the tools and build a single libpdns_common

Meson: Refactor the tools and build a single libpdns_common

libpdns_common is now shared among pdns-auth server and all its related tools. This avoids
some caveats we've previously had:

- No need for spurious builds of static archives
- No need for different libpdns_TOOLNAME libraries
- No need to list sources files multiple times
- Build everything in the top-level directory
- Avoid the race condition between pdns/, modules/ and config.h
- A much simplified pdns/meson.build which only collects source files