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/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());