safe_process.cc/safe_process.pl should not kill mysqld on SIGSTOP/SIGCONT

For Percona Server, this is prerequisite in testing bug 805805 fix in MTR.

Upstream description:

Description:
Currently safe_process.cc (and probably safe_process.pl too) handles SIGCHLD as if its child was terminated. That's the case for SIGTERM, SIGKILL etc. But safe_process also receives SIGCHLD for SIGSTOP, SIGCONT etc, which are non-fatal to the child.

How to repeat:
--source include/not_windows.inc

let $mysqld_pid_file=`SELECT @@GLOBAL.pid_file`;

system kill -STOP `cat $mysqld_pid_file`;
system kill -CONT `cat $mysqld_pid_file`;

# Server gone!
SELECT 2+2;

Suggested fix:
SIGCHLD handler should be installed with SA_NOCLDSTOP option:

@@ -152,11 +152,15 @@
   pid_t own_pid= getpid();
   pid_t parent_pid= getppid();
   bool nocore = false;
+ struct sigaction sigchld_action;
+
+ sigchld_action.sa_handler= handle_signal;
+ sigchld_action.sa_flags= SA_NOCLDSTOP;

   /* Install signal handlers */
   signal(SIGTERM, handle_signal);
   signal(SIGINT, handle_signal);
- signal(SIGCHLD, handle_signal);
+ sigaction(SIGCHLD, &sigchld_action, NULL);
   signal(SIGABRT, handle_abort);

   sprintf(safe_process_name, "safe_process[%ld]", (long) own_pid);