library/std: Rename `ON_BROKEN_PIPE_FLAG_USED` to `ON_BROKEN_PIPE_USED`
This commit is a pure internal rename and does not change any functionality.
The `FLAG_` part of `ON_BROKEN_PIPE_FLAG_USED` comes from that the compiler flag `-Zon-broken-pipe=...` is used to enable the feature.
Remove the `FLAG_` part so the name works both for the current compiler flag `-Zon-broken-pipe=...` and for the upcoming [Externally Implementable Item `#[std::io::on_broken_pipe]`](https://github.com/rust-lang/rust/pull/150591) PR. This makes the diff of that PR smaller.
The local variable name `sigpipe_attr_specified` comes from way back when the feature was controlled with an `fn main()` attribute called `#[unix_sigpipe = "..."]`. Rename that too.
cb77fcb3a6