Move loadConfFile() to initLibStore

Part of an effort to make it easier to initialize the right things,
by moving code into the appropriate libraries.

Using libstore without loading the config file is risky, as sqlite
may then be misconfigured. See cachix/cachix#475

perl/lib/Nix/Store.xs

@@ -27,7 +27,6 @@ static ref<Store> store()
     if (!_store) {
         try {
             initLibStore();
-            loadConfFile();
             settings.lockCPU = false;
             _store = openStore();
         } catch (Error & e) {

src/libmain/shared.cc

@@ -166,8 +166,6 @@ void initNix()
     if (sodium_init() == -1)
         throw Error("could not initialise libsodium");
 
-    loadConfFile();
-
     startSignalHandlerThread();
 
     /* Reset SIGCHLD to its default. */

src/libstore/globals.cc

@@ -291,6 +291,9 @@ void assertLibStoreInitialized() {
 }
 
 void initLibStore() {
+
+    loadConfFile();
+
     initLibStoreDone = true;
 }