Renaming libifc to libifconfig in response to feedback on initial commit of

Renaming libifc to libifconfig in response to feedback on initial commit of
this library. Sticking to 'libifconfig' (and 'ifconfig_' as function prefix)
should reduce chances of namespace collisions, make it more clear what the
library does, and be more in line with existing libraries.

Submitted by: Marie Helene Kvello-Aune <marieheleneka@gmail.com>
Differential Revision:	https://reviews.freebsd.org/D7742
Reviewed by:	cem, kp