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/cursor_shape.c

@@ -260,7 +260,7 @@ int cursor_mode_str2int(const char *mode)
       return current_mode;
     }
   }
-  ELOG("Unknown mode %s", mode);
+  WLOG("Unknown mode %s", mode);
   return -1;
 }