Improved logging abstraction

This also gets rid of --log-type, since the nested log type isn't
useful in a multi-threaded situation, and nobody cares about the
"pretty" log type.

src/nix/main.cc

@@ -7,6 +7,7 @@
 #include "legacy.hh"
 #include "shared.hh"
 #include "store-api.hh"
+#include "progress-bar.hh"
 
 namespace nix {
 
@@ -42,6 +43,8 @@ void mainWrapped(int argc, char * * argv)
 
     assert(args.command);
 
+    StartProgressBar bar;
+
     args.command->prepare();
     args.command->run();
 }