Add log() style versions of device_printf() and if_printf()

[adrian]

Summary

This is intended to be used by devices and network devices
to log things to the kernel syslog rather than hand-rolling
log() stuff (like what happens in other places in net/if.c)
or being very spammy with device_printf()/printf() (like, well,
a lot of things.)

I've used this in my local tree to convert net80211 and ath_hal
logging over to log at INFO/DEBUG where appropriate and it
works quite well. On devices with serial consoles (ie, all the
weird things I use in my wifi test bed here) writing debug
information to the serial console causes quite bad behaviour.

[imp]

Seems good to me, but you'll want to make sure others sign off on it too.

sys/kern/subr_bus.c

2457

128 is kinda small, but on the other hand it's also kinda big for the stack :(, but I guess it's no different than device_printf.

[jhb]

sys/kern/subr_bus.c

2457

If you wanted to support longer lines you could add a drain routine that wrote to log(). Truncating is probably fine though.

[melifaro]

That's a nice addition we should long have.
Please see some minor comments inside.

sys/kern/subr_bus.c

2462

Nit: should it be something like "unknown_device:" ?

sys/net/if.c

3974

Nit: maybe if_buf?