ifconfig_blah0="authmode wpa" causes ifconfig_getargs() to interpret

ifconfig_blah0="authmode wpa" causes ifconfig_getargs() to interpret
"wpa" as an invitation to start wpa_supplicant(8) (and to strip it
from the line) instead of just passing "authmode wpa" to ifconfig(8)
as expected.

Fix this by adding some code to ifconfig_getargs() and wpaif() which
checks if "wpa" is an argument to "authmode".

Submitted-by: Frank W. Josellis <frank@dynamical-systems.org>