setpriv: run a program with different Linux privilege settings

This new command can set no_new_privs, uid, gid, groups, securebits,
inheritable caps, the cap bounding set, securebits, and selinux and
apparmor labels.

[kerolasa@iki.fi: a lot of small adjustment making the command to be good
fit to util-linux project]

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
Signed-off-by: Andy Lutomirski <luto@amacapital.net>