Bug 4915: Detect IPv6 loopback binding errors (#355)

Bug 4915: Detect IPv6 loopback binding errors (#355)

Systems which have been partially 'IPv6 disabled' may allow
sockets to be opened and used but missing the IPv6 loopback
address.

Implement the outstanding TODO to detect such failures and
disable IPv6 support properly within Squid when they are found.

This should fix bug 4915 auth_param helper startup and similar
external_acl_type helper issues. For security such helpers are
not permitted to use the machine default IP address which is
globally accessible.