MFC r345139, r345151, r346324, r346328: ether_gen_addr KPI

if_bridge and if_vxlan conversion to this deterministic MAC address KPI has
been MFC as well. This is potentially error prone as the generated address
range for these has decreased, but I've deemed this acceptable for stable
branches due to collisions for thees interfaces being easily remedied.

I have no intention of switching anything else to this KPI in any stable
branches.

r345139:
ether: centralize fake hwaddr generation

We currently have two places with identical fake hwaddr generation --
if_vxlan and if_bridge. Lift it into if_ethersubr for reuse in other
interfaces that may also need a fake addr.

r345151:
ether_fakeaddr: Use 'b' 's' 'd' for the prefix

This has the advantage of being obvious to sniff out the designated prefix
by eye and it has all the right bits set. Comment stolen from ffec.

I've removed bryanv@'s pending question of using the FreeBSD OUI range --
no one has followed up on this with a definitive action, and there's no
particular reason to shoot for it and the administrative overhead that comes
with deciding exactly how to use it.

r346324:
net: adjust randomized address bits

Give devices that need a MAC a 16-bit allocation out of the FreeBSD
Foundation OUI range. Change the name ether_fakeaddr to ether_gen_addr now
that we're dealing real MAC addresses with a real OUI rather than random
locally-administered addresses.

r346328:
Compile sha1.c when ether support is included

sha1 is used by ether_gen_addr after r346324. Perhaps in an ideal world we
could detect that the kernel's been compiled without sha1_* bits included
and silently fallback to arc4random instead because these platforms/kernel
configs are far and few between. It's fairly lightweight, though, so just
include it for now.