Bug #36654: mysqld_multi cannot start instances with different versions

occasionally.

mysql_multi can call mysqld_safe. In doing this it's not changing the 
current working directory. This may cause confusion in the case where 
mysqld_multi is handling instances of servers of different versions 
and the current working directory is the installation directory of one 
of these servers.

Fixed by enhancing the meaning of basedir in [mysqldN] sections of 
mysqld_multi. If specified, mysqld_multi will change the current 
working directory to the basedir directory before starting the server 
in mysqld_multi ... start ... and then change it back to what it was.

scripts/mysqld_multi.sh:
  Bug #36654: optionally preserve, change and restore the cwd when 
  starting server instances

scripts/mysqld_multi.sh

 #!/usr/bin/perl
 
 use Getopt::Long;
-use POSIX qw(strftime);
+use POSIX qw(strftime getcwd);
 
 $|=1;
 $VER="2.16";
@@ -295,6 +295,7 @@ sub start_mysqlds()
   {
     @options = defaults_for_group($groups[$i]);
 
+    $basedir_found= 0; # The default
     $mysqld_found= 1; # The default
     $mysqld_found= 0 if (!length($mysqld));
     $com= "$mysqld";
@@ -310,6 +311,14 @@ sub start_mysqlds()
 	$com= $options[$j];
         $mysqld_found= 1;
       }
+      elsif ("--basedir=" eq substr($options[$j], 0, 10))
+      {
+        $basedir= $options[$j];
+        $basedir =~ s/^--basedir=//;
+        $basedir_found= 1;
+        $options[$j]= quote_shell_word($options[$j]);
+        $tmp.= " $options[$j]";
+      }
       else
       {
 	$options[$j]= quote_shell_word($options[$j]);
@@ -337,7 +346,16 @@ sub start_mysqlds()
       print "group [$groups[$i]] separately.\n";
       exit(1);
     }
+    if ($basedir_found)
+    {
+      $curdir=getcwd();
+      chdir($basedir) or die "Can't change to datadir $basedir";
+    }
     system($com);
+    if ($basedir_found)
+    {
+      chdir($curdir) or die "Can't change back to original dir $curdir";
+    }
   }
   if (!$i && !$opt_no_log)
   {