Avoid having /sbin/wifi silently ignore unknown keywords and execute
"up"; instead display the help message and exit with an error.
Spell out the "up" keyword (which has users), add it to usage output,
and preserve the implicit assumption that runing /sbin/wifi without
argument performs "up".
Signed-off-by: Thibaut VARÈNE <hacks@slashdirt.org>
78b5764fd8