ifconfig: properly detect invalid mediaopt keywords.

ifconfig: properly detect invalid mediaopt keywords.

When invalid keyword is specified, ifconfig(8) is silent about it,
instead random request is sent to the driver.

Before the patch:
root@r-freeb43:~ # ifconfig mce0 mediaopt -txpause,-rxpause
ifconfig: SIOCSIFMEDIA (media): Device not configured

After:
root@r-freeb43:~ # ifconfig mce0 mediaopt -txpause,-rxpause
ifconfig: unknown option: -txpause

Reviewed by:	hselasky, kp
Sponsored by:	Mellanox Technologies / NVidia Networking
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D27060