capability-util: add new function for raising setpcap

Up to now the capability CAP_SETPCAP was raised implicitly in the
function capability_bounding_set_drop.

This functionality is moved into a new function
(capability_gain_cap_setpcap).

The new function optionally provides the capability set as it was
before raisining CAP_SETPCAP.

(cherry picked from commit 57d4d284)
(cherry picked from commit 4f692548)