Remove VNET_SYSCTL_ARG(). The generic sysctl(9) code handles that.

Reviewed by:	ae
Sponsored by:	Nginx, Inc.
Remove useless structure ifindex_entry.

Sponsored by:	Nginx, Inc.
Sponsored by:	Netflix
Remove SYSCTL_VNET_* macros, and simply put CTLFLAG_VNET where needed.

Sponsored by:	Nginx, Inc.
libdialog has to be linked to libncursesw and libm