net: refactor if_clone.c #1

• Add ifc_find_cloner()
• Rename current ifc_find_cloner() to ifc_find_cloner_in_vnet()
• Add ifc_find_cloner_match()

This change simplifies the code a bit and reduces the diff to
the netlink interface cloners merge (D39032).

Reviewed by: glebius, kp
Differential Revision: https://reviews.freebsd.org/D39046
MFC after: 2 weeks