setpriv: allow modifying inheritable caps

setpriv: allow modifying inheritable caps

The main use case of setpriv is to modify the current state of
privileges available to the calling process and spawn a new executable
with the modified, new state. Next to the already supported case of
modifying the no-new-privs flag, util-linux also supports to modify
capability sets.

This commit introduces to add or drop capabilities from the set of
inheritable capabilities. Quoting from capabilities(7):

    This is a set of capabilities preserved across an execve(2).
    Inheritable capabilities remain inheritable when executing any
    program, and inheritable capabilities are added to the permitted set
    when executing a program that has the corresponding bits set in the
    file inheritable set.

As such, inheritable capabilities enable executing files with certain
privileges if the file itself has these privileges set. Note though that
inheritable capabilities are dropped across execve when running as a
non-root user.

function                                             old     new   delta
getcaps                                                -     237    +237
setpriv_main                                        1129    1246    +117
.rodata                                           146198  146307    +109
static.setpriv_longopts                               29      40     +11
packed_usage                                       32107   32092     -15

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>