Fix a gcc warning.

Pointy hat to:	hrs
- Fix a bug in ipv6_prefix_IF.  It did not work with the 64-bit prefix
  notation like 2001:db8:1:1.

- Use eui64 flag in ifconfig(8) instead of network6_getladdr()[*] for
  interface indentifier part.

Suggested by:	ume [*]
MFC after:	3 days