Add `PTRACE_EVENT_STOP` to `ptrace::Event`

[ranweiler]

In #749, the ptrace::Event enum was introduced, and the constant for PTRACE_EVENT_STOP was remove due to lack of support in the upstream libc crate. The reason was documented here.

Since then, rust-lang/libc#1834 added upstream support for the needed constant. For context on the libc crate's supported libc versions, see the ongoing discussion here rust-lang/libc#1412.

The upshot is that we can now reintroduce it for platforms where it is supported.

[ranweiler]

The change requested here appears to be included in an otherwise unrelated(?) open PR: #1279.

Will be closed by #1335.