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/eval.c

@@ -14361,7 +14361,7 @@ static void f_serverstart(typval_T *argvars, typval_T *rettv, FunPtr fptr)
 
   if (result != 0) {
     EMSG2("Failed to start server: %s",
-          result > 0 ? "Unknonwn system error" : uv_strerror(result));
+          result > 0 ? "Unknown system error" : uv_strerror(result));
     return;
   }