client: Add support for zone identifiers in IPv6 addresses

The IPv6 addresses with zone identifiers are refused by GVfs currently. THis is because of g_uri_unescape_segment failure as RFC 4007 allows bare % sign to be used as separator. Although, RFC 6874 tries to fix that by the %25 separator, however, at the same time, it suggests that the bare % sign should still be accepted in user interfaces. But this would make this too complex and lead to various problems (e.g. it would not be clear what separator should be used for g_file_get_uri function). So I intentionally don't plan to support what is suggested by RFC 6874 for now, which effectively means that zone identifiers with non-ASCII chars won't be supported. Let's skip the g_uri_unescape_segment function for IPv6 address and also fix the gvfs_is_ipv6 function in order to accept the zone identifiers...

Fixes: #483 (closed)