Make libifconfig INTERNALLIB

Instead of PRIVATELIB + NO_PIC. This avoids the need for the wlandebug
PIE special case added in r344211, and provides a stronger guarantee
against 3rd party software coming to depend on the API or ABI.

If / when we declare the API/ABI to be stable we can make it a normal
library.

Discussed with: bapt
Sponsored by: The FreeBSD Foundation