Backport: PR_{GET,SET}_NO_NEW_PRIVS to prevent execve from granting privs

Original message:

Add PR_{GET,SET}_NO_NEW_PRIVS to prevent execve from granting privs

With this set, a lot of dangerous operations (chroot, unshare, etc)
become a lot less dangerous because there is no possibility of
subverting privileged binaries.

This patch completely breaks apparmor.  Someone who understands (and
uses) apparmor should fix it or at least give me a hint.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>

Change-Id: I370e8cc0fd6046c0b733c35447698e4e4531454f
(cherry picked from commit d803f7a504a0522ab4d81a6e37b50d3df8985da6)