make "ifconfig <if> inet6 eui64" reset the NOINET6 flag
(unconditionally), so a link-local will be assigned if there isn't one
yet. ok krw benno todd sthen
SystemMemory is used to access memory mapped registers on some machines, so we
must use the bus_space(9) API to access it instead of memcpy(9).  Also make
sure we properly align access to fields that need more than byte alignment.

ok guenther@
turn of IPv6 for new interfaces by default. this really just means there
is no link-local address assigned by default, which - by default, since
INET6 is in the GENERIC kernels - makes the machine and services on it
reachable. No such thing in IPv4 land.
since assigning an inet6 address manually or running rtsol(d) etc resets the
NOINET6 flag and causes the link-local to be assigned, this really is a noop
for all inet6 users with the exception of those ONLY using the default
link-local address. These needs to run "ifconfig <if> inet6 eui64" or put
the equivalent in hostname.if.
discussed at length with many, ok at least by krw todd benno sthen
Fix possible crash on invalid filesystems.
Merged from FreeBSD's revision 206098.

ok beck@
first attempt at documenting NOINET6 by default and eui64 turning it on again