Ensure that link-local addresses are correctly configured on loopback

Ensure that link-local addresses are correctly configured on loopback
interfaces.

When the kernel automagically configures IPv6 addresses on loopback
interfaces, start by assigning a link-local address and then try to
assign "::1".

Only the first configured loopback interface per rdomain can have the
"::1" address.  But even if other loopback interfaces failed to get
this address, because it is already taken, give them a chance to have
a link-local address.

While here change in6_ifattach() to return an error value and remove
duplicated code.

Fix a regression introduced by the NOINET6 flag removal.

ok henning@, stsp@, florian@, benno@