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