util: add a vercmp function to compare version numbers

It is simple, supports simple x.y.z type numeric versions,
and does not attempt any kind of validation

src/common/util.h

@@ -183,4 +183,5 @@ namespace tools
   unsigned get_max_concurrency();
 
   bool is_local_address(const std::string &address);
+  int vercmp(const char *v0, const char *v1); // returns < 0, 0, > 0, similar to strcmp, but more human friendly than lexical - does not attempt to validate
 }