Use the new host_str* functions to improve IPv6 literal support.

Use the new host_str* functions to improve IPv6 literal support.

I've gone through everywhere we handle host names / addresses (on
command lines, in PuTTY config, in port forwarding, in X display
names, in host key storage...) and tried to make them handle IPv6
literals sensibly, by using the host_str* functions I introduced in my
previous commit. Generally it's now OK to use a bracketed IPv6 literal
anywhere a hostname might have been valid; in a few cases where no
ambiguity exists (e.g. no :port suffix is permitted anyway)
unbracketed IPv6 literals are also acceptable.

[originally from svn r10120]