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.

src/basic/capability-util.h

@@ -14,6 +14,7 @@
 
 unsigned cap_last_cap(void);
 int have_effective_cap(int value);
+int capability_gain_cap_setpcap(cap_t *return_caps);
 int capability_bounding_set_drop(uint64_t keep, bool right_now);
 int capability_bounding_set_drop_usermode(uint64_t keep);