Revisit IPv4/IPv6 settings and behavior in pjsua

[ming]

Currently, in the account settings, SIP and media has to be predetermined whether it wants to use a particular IP version, IPv4 or IPv6. As a result, typically two accounts are required for each IP version, one for IPv4, and one for IPv6.

It is desirable that a more flexible/smart detection mechanism is employed, such as:

• Ability to handle both IP versions in SIP and media, especially in the case of incoming offer where we can check what the remote wants.
• Specify a preference of one IP version over the other, but still can handle both.
• Match the IP version for SIP and media, i.e. if SIP signalling is done via IPv6, then media will also use IPv6.

Thanks to Alexander Traud for the suggestion.