Auto merge of #101077 - sunshowers:signal-mask-inherit, r=sunshowers

Change process spawning to inherit the parent's signal mask by default

Previously, the signal mask was always reset when a child process is
started. This breaks tools like `nohup` which expect `SIGHUP` to be
blocked for all transitive processes.

With this change, the default behavior changes to inherit the signal mask.

This also changes the signal disposition for `SIGPIPE` to only be changed if the `#[unix_sigpipe]` attribute isn't set.