setpriv: proxy function checking whether a capability is set

setpriv: proxy function checking whether a capability is set

The loop in `print_caps` iterates over every capability, checks whether
it is set and, if so, prints out its name. Currently, the checking and
printing is rather intertwined, making it harder to extend the check
whether we own a capability.

Prepare code for the introduction of ambient capabilities by
disentangling the code checking for a capability and printing code. A
new function `has_cap` is introduced and `print_caps` will now simply
call out to it and only handle printing itself. This easily allows to
extend the capability check based on which capability set is queried.

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