Become process group leader when updating firmware

As exec is invoked, child process can not ignore the signal sent
to the process group, use setpgrp to set the pgid to itself.

close-issue: #1849

xCAT-server/lib/xcat/plugins/ipmi.pm

@@ -1924,6 +1924,11 @@ sub do_rflash_process {
 
     # child
     elsif ($pid == 0) {
+        unless (setpgrp()) {
+            xCAT::SvrUtils::sendmsg([ 1, "Faild to run setgprp for $$ process" ],
+                $callback, $node);
+            exit(1);
+        }
         my $extra = $_[8];
         my @exargs = @$extra;
         my $programe = \$0;