log: Always enable; remove DISABLE_LOG

- Establish ERROR log level as "critical". Such errors are rare and will
  be valuable when users encounter unusual circumstances.
- Set -DMIN_LOG_LEVEL=3 for release-type builds

src/nvim/os/shell.c

@@ -685,7 +685,7 @@ static void shell_write_cb(Stream *stream, void *data, int status)
                        uv_err_name(status));
   }
   if (stream->closed) {  // Process may have exited before this write.
-    ELOG("stream was already closed");
+    WLOG("stream was already closed");
     return;
   }
   stream_close(stream, NULL, NULL);