ifconfig: add if_ctx argument to the generic and ifclone callbacks.

This is the continuation of the ifconfig cleanup work. This change is
a pre-requsite for the next changes removing some of the global variables.
It will also help in implementing functionality via Netlink instead of ioctl.
No functional changes intended.

• vxlan_cb() was removed as it contained no code
• ioctl_ifcreate() was renamed to ifcreate_ioctl() to follow the other

netlink/ioctl function naming. Netlink and ioctl provide _different_
interfaces and it's not possible to have a unified interface object
that can be filled by either netlink or ioctl implementations. With that
in mind, I'm leaning more to the function_<nl|ioctl> postfix pattern,
than doing ioctl_ or netlink_ prefix.

Reviewed By: kp
Differential Revision: https://reviews.freebsd.org/D40426
MFC after: 2 weeks