Fix freeze when launching web browser on Linux/macOS

POSIX has a feature to automatically reap zombie processes, which makes it
unnecessary to wait for the child process to exit. Activate the feature
and stop waiting for exit.

"POSIX.1-2001 specifies that if the disposition of SIGCHLD is set to
SIG_IGN or the SA_NOCLDWAIT flag is set for SIGCHLD (see sigaction(2)),
then children that terminate do not become zombies"

src/desktop/open.cpp

@@ -73,11 +73,11 @@ bool open_object(const std::string& path_or_url)
 	} else if(child == 0) {
 		execlp(launcher, launcher, path_or_url.c_str(), nullptr);
 		_exit(1); // This shouldn't happen.
-	} else if(waitpid(child, &child_status, 0) == -1) {
-		ERR_DU << "open_object(): waitpid() failed" << std::endl;
-		return false;
 	}
 
+	// Waiting for the child process to exit is unnecessary because we ignore SIGCHLD.
+	// See the manpage for wait(2).
+
 	if(child_status) {
 //Those status check macros seem to trigger old style casts on some compiler versions
 #pragma GCC diagnostic push

src/wesnoth.cpp

@@ -1015,6 +1015,11 @@ int main(int argc, char** argv)
 	sigemptyset(&terminate_handler.sa_mask);
 	sigaction(SIGTERM, &terminate_handler, nullptr);
 	sigaction(SIGINT, &terminate_handler, nullptr);
+
+	// Explicitly ignore SIGCHLD. This allows us to launch child processes without waiting
+	// for them to exit. See the manpage for wait(2).
+	terminate_handler.sa_handler = SIG_IGN;
+	sigaction(SIGCHLD, &terminate_handler, nullptr);
 #endif
 
 	//declare this here so that it will always be at the front of the event queue.