basic/capability-util: let cap_last_cap() return unsigned integer

We never return anything higher than 63, so using "long unsigned"
as the type only confused the reader. (We can still use "long unsigned"
and safe_atolu() to parse the kernel file.)

src/basic/capability-util.h

@@ -12,7 +12,7 @@
 
 #define CAP_ALL (uint64_t) -1
 
-unsigned long cap_last_cap(void);
+unsigned cap_last_cap(void);
 int have_effective_cap(int value);
 int capability_bounding_set_drop(uint64_t keep, bool right_now);
 int capability_bounding_set_drop_usermode(uint64_t keep);