NormalizeCap lowercases a capability name and strips any "cap_" prefix,
accepting all four common formats: "net_bind_service", "NET_BIND_SERVICE",
"cap_net_bind_service", "CAP_NET_BIND_SERVICE".
ParseCap resolves a capability name to its kernel integer value. Names are
normalized before lookup: case-insensitive and with or without the "cap_"
prefix, so "NET_BIND_SERVICE", "cap_net_bind_service", "CAP_NET_BIND_SERVICE",
and "net_bind_service" all resolve to the same value.