launcher-service: Unblock and unignore all signals during startup
Steam could run this tool in an arbitrary environment, with necessary signals either blocked (sigprocmask() or similar), or ignored (sigaction() with action SIG_IGN, or similar). Both of these are inherited across fork() and execve() by default, and current versions of Steam do block SIGCHLD in practice. However, blocking or ignoring SIGCHLD breaks handling of subprocesses, which is particularly critical for s-r-launcher-service: that prevents it from detecting that its child processes have exited, causing s-r-launch-client to hang forever, which in turn breaks startup of SteamVR on systems where /bin/sh is dash (but not bash, which apparently resets the disposition of SIGCHLD internally). steamrt/tasks#354, https://github.com/ValveSoftware/SteamVR-for-Linux/issues/618 Signed-off-by:Simon McVittie <smcv@collabora.com>