lib/caputils: add fall back for last cap using prctl.

lib/caputils: add fall back for last cap using prctl.

This allows the rest of the programs using cap_last_cap to trust the
value returned by it, since it will either be obtained from procfs
(straight from kernel) or with prctl.

Also checked if the file under /proc is actually mounted in a procfs.

(cherry picked from commit 5d95818757941bc609e5aeec5e2218f7d35a6e19)