Block signals during fpm master initialization
Fix PHP-FPM failure in the case of concurrent reload attempts.
Postpone signal delivery to the fpm master process till proper signal
handlers are set. Prevent the following case:
- Running master process receives `SIGUSR2` and performs `execvp()`.
- Another `SIGUSR2` is arrived before signal handlers are set.
- Master process dies.
- Requests to the HTTP server handled by PHP-fpm can not be served
any more.
Block some signals using `sigprocmask()` before `execvp()` and early
in the `main()` function. Unblock signals as soon as proper
handlers are set.
Fixes bug #74083