restore SIGCHLD settings to SIG_DFL. #600

[masm11]

I startup "Terminator", a terminal emulator, from Waybar button.

Waybar forks on button click, and the child process executes Terminator,
and the Waybar process itself setups SIGCHLD to SIG_IGN to discard defunct child processes.
But the SIGCHLD setting is inherited to a new child process on fork when I click the button next time.

That is why Terminator startups with SIGCHLD setting of SIG_IGN on the second click.

Terminator forks, executes uname -p, and waits finishing of the uname -p process.
But because Terminator's SIGCHLD setting is SIG_IGN, uname -p process disappears just after it exits,
and Terminator can't wait it, raise an exeception, and exits.

The solution is to restore SIGCHLD setting of child processes to SIG_DFL after forks.
Then, Terminator's SIGCHLD setting becomes SIG_DFL, and uname -p process doesn't disappears until
Terminator waits.

[Alexays]

Thanks!