Tie ifmedia to the generic framework of new ifnet KPI.

Implementation:
o struct if_media goes opaque into if_media.c
o ifmedia is an optional softc (keyword IF_MEDIA) of an interface.
o if_media.h declares just preprocessor defines to manipulate media

bits. Since a conservative decision was taken in r281236, the media
information remains a single 32-bit word.
However, to improve type checking it is declared as if_media_t, not
a bare integer.
if_media.h is now automatically included from if.h

o if_attach() calls ifmedia_alloc(). if_free() calls ifmedia_free().

Driver changes:
o Drivers supply ifop_media_change and ifop_media_status callbacks in

their ifdriver ifops. This were arguments to ifmedia_init() before.
The ifop_media_change() callback gets additional argument of the new
media, we are trying to swtich to.

o On attach, drivers supply array of supported media words in ifat_mediae,

currently chose media in ifat_media and optional ifat_mediamask.
Drivers may change the media array later via if_media_change().

o Drivers don't need to support SIOCSIFMEDIA, SIOCGIFMEDIA. The stack

does now.

o Drivers may call if_media_status() if they are running autonegotiation

and wish to report new status.  The function updates link state and
baudrate of the interface.

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