MDEV-25177 Better indication of refusing to start because of ProtectHome

Create test for for case insensitive gives a basic warning on creating
a test file and the next thing a user might see is an abort.

ProtectHome and other systemd setting protect system services from
accessing user data. Unfortunately some of our users do put things
on /home due space or other reasons.

Rather than enumberate the systemd options in a very clunkly fragile
way we put an error associated with the "Can't create test file" and
hope the user can work it out from there.

%M tip thanks Sergei.

sql/mysqld.cc

@@ -9299,7 +9299,7 @@ static int test_if_case_insensitive(const char *dir_name)
                                buff, 0666, O_RDWR, MYF(0))) < 0)
   {
     if (!opt_abort)
-      sql_print_warning("Can't create test file %s", buff);
+      sql_print_warning("Can't create test file '%s' (Errcode: %M)", buff, my_errno);
     DBUG_RETURN(-1);
   }
   mysql_file_close(file, MYF(0));