Modules determine their own compatibility with the library

A module is built against the system-installed libdpm-core.so and is
responsible for being correct against it. Where it needs to act on the
version it is running under, dpm_core_version() reports that and the
module decides for itself.

dpm_module_core_min() is removed. It was a declaration handed to the
library to enforce on the module's behalf, and enforcement of that kind
belongs nowhere in a library that routes and hosts. The contract is now
three reserved symbols and load validation is two steps: the reserved
symbols resolve, and the version and description probes return
well-formed values.

compare_versions had no remaining caller and is removed; parse_version
stays for the well-formedness probe. The core_too_new fixture went with
the handshake it existed to exercise.

include/internal/modules.hpp

@@ -34,7 +34,6 @@ struct dpm_module {
     void*       handle = nullptr;
     std::string version;
     std::string description;
-    std::string core_min;
     int (*execute)(dpm_ctx*, const char*, int, char**) = nullptr;
 };