IPv6 support in PJSIP

[bennylp]

Modifications that are needed in PJSIP to support IPv6:

• parsing and generation of IPv6 values in SIP messages (see ​http://tools.ietf.org/html/draft-gurbani-sipping-ipv6-sip-03)
• support for IPv6 SIP transports (UDP, TCP, and TLS)
• DNS AAAA resolution in DNS SRV resolution for server names

[bennylp]

Initial UDP over IPv6 implementation in r1602.

Details:

• Implemented IPv6 UDP transport:
• New transport type: PJSIP_TRANSPORT_UDP6 and PJSIP_TRANSPORT_TCP6
• New API: pjsip_transport_type_get_af()
• New API: pjsip_transport_get_type_desc()
• New API: pjsip_udp_transport_start6()
• New API: pjsip_udp_transport_attach2()

[bennylp]

In r1610:

• Fixed SIP messaging components (parsing and message representation) to support IPv6