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/libmain/common-args.cc

@@ -18,15 +18,6 @@ MixCommonArgs::MixCommonArgs(const string & programName)
         verbosity = lvlDebug;
     });
 
-    mkFlag1(0, "log-type", "type", "set logging format ('pretty', 'flat', 'systemd')",
-        [](std::string s) {
-            if (s == "pretty") logType = ltPretty;
-            else if (s == "escapes") logType = ltEscapes;
-            else if (s == "flat") logType = ltFlat;
-            else if (s == "systemd") logType = ltSystemd;
-            else throw UsageError("unknown log type");
-        });
-
     mkFlag(0, "option", {"name", "value"}, "set a Nix configuration option (overriding nix.conf)", 2,
         [](Strings ss) {
             auto name = ss.front(); ss.pop_front();

src/libmain/shared.cc

@@ -111,8 +111,7 @@ void initNix()
     std::cerr.rdbuf()->pubsetbuf(buf, sizeof(buf));
 #endif
 
-    if (getEnv("IN_SYSTEMD") == "1")
-        logType = ltSystemd;
+    logger = makeDefaultLogger();
 
     /* Initialise OpenSSL locking. */
     opensslLocks = std::vector<std::mutex>(CRYPTO_num_locks());