nix doctor: add command

Inspired by the homebrew command, shows a combination of debugging
information and warnings with potential issues with a nix installation.

src/nix/doctor.cc

@@ -0,0 +1,26 @@
+#include "command.hh"
+#include "shared.hh"
+#include "store-api.hh"
+
+using namespace nix;
+
+struct CmdDoctor : StoreCommand
+{
+    std::string name() override
+    {
+        return "doctor";
+    }
+
+    std::string description() override
+    {
+        return "check your system for potential problems";
+    }
+
+    void run(ref<Store> store) override
+    {
+        std::cout << "Store uri: " << store->getUri() << std::endl;
+    }
+};
+
+static RegisterCommand r1(make_ref<CmdDoctor>());
+