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.

tests/fixtures/src/bad_version.cpp

@@ -30,11 +30,6 @@ extern "C" const char* dpm_module_description(void)
     return "Fixture with a malformed version.";
 }
 
-extern "C" const char* dpm_module_core_min(void)
-{
-    return "0.1.0";
-}
-
 extern "C" int dpm_module_execute(void* ctx, const char* command,
                                   int argc, char** argv)
 {