We don't really care if the process has a custom handler installed, we
can just set it to default.
The one we check is SIGHUP, which may be given to us via `nohup`.
This saves ~30 syscalls *per process* we spawn, so:
```fish
for f in (seq 1000)
    command true
end
```
has ~30000 fewer rt_sigaction calls. These take up about ~30% of the
total time spent in syscalls according to strace.
We could also compute this set once at startup and then reuse it.

faho added the performance

label

faho added a commit that referenced this pull request

Really the only thing we're looking for here is if we're started with
HUP ignored or not.
Saves a syscall per external process.
Continuation of #10869

github-actions Bot locked as resolved and limited conversation to collaborators