Implement SIOCSIFLLADDR, which allows you to change the link-level

address on an interface. This basically allows you to do what my
little setmac module/utility does via ifconfig. This involves the
following changes:

socket.h: define SIOCSIFLLADDR
if.c: add support for SIOCSIFLLADDR, which resets the values in
      the arpcom struct and sockaddr_dl for the specified interface.
      Note that if the interface is already up, we need to down/up
      it in order to program the underlying hardware's receive filter.
ifconfig.c: add lladdr command
ifconfig.8: document lladdr command

You can now force the MAC address on any ethernet interface to be
whatever you want. (The change is not sticky across reboots of course:
we don't actually reprogram the EEPROM or anything.) Actually, you
can reprogram the MAC address on other kinds of interfaces too; this
shouldn't be ethernet-specific (though at the moment it's limited to
6 bytes of address data).

Nobody ran up to me and said "this is the politically correct way to
do this!" so I don't want to hear any complaints from people who think
I could have done it more elegantly. Consider yourselves lucky I didn't
do it by having ifconfig tread all over /dev/kmem.

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