Add some hard-coded textual literal-IP representations of localhost to

Add some hard-coded textual literal-IP representations of localhost to
sk_hostname_is_local(), to catch the case where we're doing something like X11
forwarding over SSH through a proxy, and we've thus disabled local lookup of
hostnames.
(I think this is what's behind the report in
<e9a86996-5dc2-4428-9b0c-c65693ca6351@m32g2000hsf.googlegroups.com>
in comp.security.ssh, although I'd like to know more of the circumstances.)

[originally from svn r8385]