Cook: security things in Linux v5.1

Kees Cook reviewsthe security-related enhancements in the 5.1 kernel release."Now /proc/$pid can be opened and used as an argument for sendingsignals with the new pidfd_send_signal() syscall. This handle will onlyrefer to the original process at the time the open() happened, and not toany later 'reused' pid if the process dies and a new process is assignedthe same pid. Using this method, it's now possible to racelessly sendsignals to exactly the intended process without having to worry about pidreuse. (BTW, this commit wins the 2019 award for Most Well DocumentedCommit Log Justification.)"