setpriv: allow using [-+]all for capabilities.

setpriv: allow using [-+]all for capabilities.

The initial change to lib/caputils that allowed this was commit
5d95818757941bc609e5aeec5e2218f7d35a6e19, which made it possible to
trust the value returned by cap_last_cap().

The error message was also somewhat misleading, since cap_last_cap()
being smaller than CAP_LAST_CAP happens when setpriv itself is built
with kernel headers older than the currently running kernel, not due to
libcap-ng.

(cherry picked from commit 93de9f687d1640fff963f26b7db474eef3746532)