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]

unix/uxputty.c

@@ -76,8 +76,7 @@ int process_nonoption_arg(char *arg, Conf *conf, int *allow_launch)
             q += 2;
         conf_set_int(conf, CONF_protocol, PROT_TELNET);
         p = q;
-        while (*p && *p != ':' && *p != '/')
-            p++;
+        p += host_strcspn(p, ":/");
         c = *p;
         if (*p)
             *p++ = '\0';