libmisc: retain setfcap when mapping uid 0

libmisc: retain setfcap when mapping uid 0

When uid 0 maps host uid 0 into the child userns newer kernels require
CAP_SETFCAP be retained as this allows the caller to create fscaps that
are valid in the ancestor userns. This was a security issue (in very
rare circumstances). So whenever host uid 0 is mapped, retain
CAP_SETFCAP if the caller had it.
Userspace won't need to set CAP_SETFCAP on newuidmap as this is really
only a scenario that real root should be doing which always has
CAP_SETFCAP. And if they don't then they are in a locked-down userns.
(LXC sometimes maps host uid 0 during chown operations in a helper
 userns but will not rely on newuidmap for that. But we don't want to
 risk regressing callers that want to rely on this behavior.)

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>