MDEV-21039: Server fails to start with unknown mysqld_safe options

Adding any unknown option to the "[mysqld_safe]" section makes
mysqld impossible to start with mysqld_multi. For example, after
adding the unknown option "numa_interleave" to the "[mysqld_safe]"
section, mysqld_multi exits with the following diagnostics:

[ERROR] /usr/local/mysql/bin/mysqld: unknown option '--numa_interleave'

To get rid of this behavior, this patch by default adds the "--loose-"
prefix to all unknown (for mysqld_safe) options. This behavior can be
enabled explicitly with the --ignore-unknown option and disabled with
the --no-ignore-unknown option.

scripts/mysqld_safe.sh

@@ -24,6 +24,7 @@ unsafe_my_cnf=0
 wsrep_on=0
 dry_run=0
 defaults_group_suffix=
+ignore_unknown=1
 
 # Initial logging status: error log is not open, and not using syslog
 logging=init
@@ -383,11 +384,22 @@ parse_arguments() {
 
       --help) usage ;;
 
+      --ignore-unknown) ignore_unknown=1 ;;
+      --no-ignore-unknown|--not-ignore-unknown) ignore_unknown=0 ;;
+
       *)
+        if test $ignore_unknown -eq 0
+        then
           case "$unrecognized_handling" in
             collect) append_arg_to_args "$arg" ;;
-          complain) log_error "unknown option '$arg'" ;;
+            complain) log_error "unknown option '$arg'"
+          esac
+        else
+          case "$arg" in
+            "--loose-"*) append_arg_to_args "$arg" ;;
+            *)           append_arg_to_args "--loose-$arg"
           esac
+        fi
     esac
   done
 }