Do not let PHP-FPM children miss SIGTERM, SIGQUIT

Postpone signal delivery while spawning children.
Prevent the following case:

- Reload (reexec) is in progress.
- New master is forking to start enough children for pools
  where `pm` is not `on-demand`.
- Another `SIGUSR2` is received by the master process.
- Master process switches to reloading state.
- Some child has not set its own signal handlers.
- `SIGQUIT` and `SIGTERM` sent by master process are caught
  by signal handler set by master process and so they are ignored.
- A child is running, it has no reason to finish

Before pull request #4465 this scenario could cause deadlock,
however with 0ed6c37140 reload finishes after `SIGKILL`.

Use sigprocmask() around fork() to avoid race of delivery signal to children
and setting of own signal handlers.

Fixes bug #76601

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