Add "warning" verbosity level

This ensures that "nix" shows warnings. Previously these were hidden
because they were at "info" level.

src/libutil/logging.cc

@@ -21,7 +21,7 @@ Logger * logger = makeDefaultLogger();
 
 void Logger::warn(const std::string & msg)
 {
-    log(lvlInfo, ANSI_RED "warning:" ANSI_NORMAL " " + msg);
+    log(lvlWarn, ANSI_RED "warning:" ANSI_NORMAL " " + msg);
 }
 
 class SimpleLogger : public Logger
@@ -46,6 +46,7 @@ public:
             char c;
             switch (lvl) {
             case lvlError: c = '3'; break;
+            case lvlWarn: c = '4'; break;
             case lvlInfo: c = '5'; break;
             case lvlTalkative: case lvlChatty: c = '6'; break;
             default: c = '7';

src/libutil/logging.hh

@@ -6,6 +6,7 @@ namespace nix {
 
 typedef enum {
     lvlError = 0,
+    lvlWarn,
     lvlInfo,
     lvlTalkative,
     lvlChatty,

src/nix/main.cc

@@ -102,7 +102,7 @@ void mainWrapped(int argc, char * * argv)
         if (legacy) return legacy(argc, argv);
     }
 
-    verbosity = lvlError;
+    verbosity = lvlWarn;
     settings.verboseBuild = false;
     evalSettings.pureEval = true;