MDEV-36206: Fix mysqld deprecation warning

When running mariabackup with --prepare --export options, a deprecation
warning appears because the program name is set to "mysqld" instead of "mariadbd"

The fix ensures that when running in mysqld mode, we properly set the
program name to "mariadbd" to avoid the deprecation warning while
maintaining the original functionality

extra/mariabackup/xtrabackup.cc

@@ -2119,7 +2119,7 @@ static int prepare_export()
   if (strncmp(orig_argv1,"--defaults-file=", 16) == 0)
   {
     snprintf(cmdline, sizeof cmdline,
-      IF_WIN("\"","") "\"%s\" --mysqld \"%s\""
+      IF_WIN("\"","") "\"%s\" --mariadbd \"%s\""
       " --defaults-extra-file=./backup-my.cnf --defaults-group-suffix=%s --datadir=."
       " --innodb --innodb-fast-shutdown=0 --loose-partition"
       " --innodb-buffer-pool-size=%llu"
@@ -2133,7 +2133,7 @@ static int prepare_export()
   else
   {
     snprintf(cmdline, sizeof cmdline,
-      IF_WIN("\"","") "\"%s\" --mysqld"
+      IF_WIN("\"","") "\"%s\" --mariadbd"
       " --defaults-file=./backup-my.cnf --defaults-group-suffix=%s --datadir=."
       " --innodb --innodb-fast-shutdown=0 --loose-partition"
       " --innodb-buffer-pool-size=%llu"
@@ -7528,9 +7528,9 @@ int main(int argc, char **argv)
 	{
 		/* In "prepare export", we need  to start mysqld 
 		Since it is not always be installed on the machine,
-		we start "mariabackup --mysqld", which acts as mysqld
+		we start "mariabackup --mariadbd", which acts as mysqld
 		*/
-		if (strcmp(argv[1], "--mysqld") == 0)
+		if (strcmp(argv[1], "--mariadbd") == 0)
 		{
 			srv_operation= SRV_OPERATION_EXPORT_RESTORED;
 			extern int mysqld_main(int argc, char **argv);