MDEV-19211 Fix mysqld_safe --dry-run

mysqld_safe --dry-run needs to either call exit or return, depending if
it is being sourced or not, otherise return can lead to the error:

return: can only `return' from a function or sourced script

The original fix suggestion was proposed by FaramosCZ <mschorm@centrum.cz>

scripts/mysqld_safe.sh

@@ -970,7 +970,13 @@ do
   cmd="$cmd "`shell_quote_string "$i"`
 done
 cmd="$cmd $args"
-[ $dry_run -eq 1 ] && return
+
+if [ $dry_run -eq 1 ]
+then
+  # RETURN or EXIT depending if the script is being sourced or not.
+  (return 2> /dev/null) && return || exit
+fi
+
 
 # Avoid 'nohup: ignoring input' warning
 test -n "$NOHUP_NICENESS" && cmd="$cmd < /dev/null"