fts: Fix double-free with conflicting concurrent modifications.

If rare conditions such as concurrent conflicting manipulation of the
filesystem occur, fts_read() frees the current FTSENT without adjusting
the pointers in the FTS accordingly. A later fts_close() then frees the
same FTSENT again.

Reported by:	pho
Tested by:	pho
MFC after:	1 week
Remove identical vnet sysctl handlers, and handle CTLFLAG_VNET
in the sysctl_root().

Note: SYSCTL_VNET_* macros can be removed as well. All is
  needed to virtualize a sysctl oid is set CTLFLAG_VNET on it.
  But for now keep macros in place to avoid large code churn.

Sponsored by:	Nginx, Inc.