userns: Allow PR_CAPBSET_DROP in a user namespace.

As the capabilites and capability bounding set are per user namespace
properties it is safe to allow changing them with just CAP_SETPCAP
permission in the user namespace.

Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Tested-by: Richard Weinberger <richard@nod.at>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>