When the `Pid` type changed to only accept positive pid values,
the `waitpid` function became unable to wait for process groups,
because that involves negative pids. Fully fixing this will
require a semver bump, but for now, we can add a new `waitpgid`
function to add support for waiting for a process group.

Also add a `Pgid` arm to `WaitId`, to support waiting for process
groups from `waitid` as well.