Add lvlNotice log level

This is like syslog's LOG_NOTICE: "normal, but significant,
condition".

src/libutil/error.hh

@@ -45,6 +45,7 @@ namespace nix {
 typedef enum {
     lvlError = 0,
     lvlWarn,
+    lvlNotice,
     lvlInfo,
     lvlTalkative,
     lvlChatty,

src/libutil/logging.hh

@@ -198,6 +198,7 @@ extern Verbosity verbosity; /* suppress msgs > this */
     } while (0)
 
 #define printError(args...) printMsg(lvlError, args)
+#define notice(args...) printMsg(lvlNotice, args)
 #define printInfo(args...) printMsg(lvlInfo, args)
 #define printTalkative(args...) printMsg(lvlTalkative, args)
 #define debug(args...) printMsg(lvlDebug, args)