Implement send_signal for unix child processes
Tracking issue: rust-lang/rust#141975
There are two main differences between my implementation and the Public API section of the tracking issue. ~First, `send_signal` requires a mutable reference, like `Child::kill`.~ Second, `ChildExt` has `Sealed` as a supertrait, bringing it more in line with other extension traits like `CommandExt`.
try-job: `dist-various*`
try-job: `test-various*`
e381a14b7c