MDEV-7364 - mysqld --help --verbose prints random values for "debug"

better fix that doesn't make debug-dbug variable to disappear
from mysqld --help output (this fact is used by have_debug.inc)
and shows correctly what dbug value was set on the command line.

sql/mysqld.cc

@@ -1418,7 +1418,7 @@ my_bool plugins_are_initialized= FALSE;
 #ifndef DBUG_OFF
 static const char* default_dbug_option;
 #endif
-static const char *current_dbug_option="disabled";
+const char *current_dbug_option="";
 #ifdef HAVE_LIBWRAP
 const char *libwrapName= NULL;
 int allow_severity = LOG_INFO;
@@ -8641,6 +8641,7 @@ mysqld_get_one_option(int optid, const struct my_option *opt, char *argument)
     if (argument[0] == '1' && !argument[1])
       break;
     DBUG_SET_INITIAL(argument);
+    current_dbug_option= argument;
     opt_endinfo=1;				/* unireg: memory allocation */
 #else
     sql_print_warning("'%s' is disabled in this build", opt->name);

sql/mysqld.h

@@ -105,6 +105,7 @@ extern my_bool read_only, opt_readonly;
 extern my_bool lower_case_file_system;
 extern my_bool opt_enable_named_pipe, opt_sync_frm, opt_allow_suspicious_udfs;
 extern my_bool opt_secure_auth;
+extern const char *current_dbug_option;
 extern char* opt_secure_file_priv;
 extern char* opt_secure_backup_file_priv;
 extern size_t opt_secure_backup_file_priv_len;

sql/sys_vars.h

@@ -742,7 +742,7 @@ class Sys_var_dbug: public sys_var
                on_update_function on_update_func=0,
                const char *substitute=0)
     : sys_var(&all_sys_vars, name_arg, comment, flag_args,
-              - (ptrdiff_t) &global_system_variables, getopt.id,
+              (char*)&current_dbug_option-(char*)&global_system_variables, getopt.id,
               getopt.arg_type, SHOW_CHAR, (intptr)def_val,
               lock, binlog_status_arg, on_check_func, on_update_func,
               substitute)