wpa_supplicant: Handle systems without INET (legacy IP) support

Currently, wpa_supplicant fails when legacy IP support is disabled
in FreeBSD (i.e., the world built with WITHOUT_INET and nooptions
INET in the kernel config). The proposed patch allows running
wpa_supplicant and connecting to wireless networks without INET
support when INET6 is available.

Reviewed by: cy, adrian, philip, roy_marples.name
Differential Revision: https://reviews.freebsd.org/D49959

(cherry picked from commit 8c7149c73f8f2301369f271c98470b72973b0c01)