Fix broken CTLFLAG_SKIP when present on the first child of the requested node.

We don't need to ignore skip for the first node because in sysctl_all() we've implicitly visited the first node already when oid is specified. The first call to show_var() in here is after we have iterated to the next node. When the command line specifically requests a non-node sysctl we go straight into show_var() without calling sysctl_all().

Reported by: jhb
Sponsored by: iXsystems, Inc.