CFR: if_media changes for multi-mode devices

[Sam Leffler]

The patch:

http://people.freebsd.org/~sam/ifmedia.diff

has changes to support multi-mode devices such as 11a/b/g wireless cards.
These use 3 of the free bits in the media word to specifiy an optional mode
that devices can honor.  To manipulate this field there is a new "mode"
command line option to ifconfig.  For example,

ifconfig ath0 mode 11g

operate my multi-mode Atheros wireless device in 11g mode.

Other changes in this diff add a "turbo" option for enabling turbo mode on
802.11 devices, add 802.11 subtype options for specifying 11g rates (CCK at
6,9,12,18,24,36,48, ad 54 Mb/s), and correct the spelling of OFDM.

These changed should not affect existing code.  If I don't hear anything
negative back in a few days I'll commit it.

    Sam