setpriv: proxy function to update capabilities

setpriv: proxy function to update capabilities

libcap-ng provides a function to update capabilities with
`capng_update`. As libcap-ng has not yet been updated to enable
modification of ambient capabilities, we cannot use it to update this
set, though. In order to allow easily extending the logic to also handle
ambient capability sets, we create a new function `cap_update`. Right
now, it simply calls out to `capng_update` for all supported capability
types.

Reviewed-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Patrick Steinhardt <ps@pks.im>