IPv6 support in PJNATH

[bennylp]

Modifications that are needed to support IPv6 in PJNATH:

• Support for IPv6 in ICE

Notes:

Refer to #1933 for pjnath-test update

[nanang]

In 5339:

Re #422: Added IPv6 support to PJNATH, changes:

• Deprecated 'pj_ice_strans_cfg.af', if set, the value will be ignored, address family setting is now specified via transport setting, i.e: 'pj_ice_strans_cfg.stun_tp/turn_tp'.
• Deprecated 'pj_ice_strans_cfg.stun/turn', for backward compatibility, this field value will be checked if 'pj_ice_strans_cfg.stun_tp_cnt/turn_tp_cnt' is set to zero.
• Added 'pj_ice_strans_stun_cfg' & 'pj_ice_strans_stun_cfg' and the corresponding 'pj_ice_strans_stun/turn_cfg_default()'
• Added 'pj_ice_strans_cfg.stun_tp/turn_tp' as replacement of 'pj_ice_strans_cfg.stun/turn', it is now an array so app can have multiple STUN/TURN transports.
• Added macro PJ_ICE_MAX_STUN/TURN to specify maximum number of STUN/TURN transports in each ICE component in compile-time.
• Miscellaneous: fixed socket number limit in concurrency test in pjnath-test, updated pjsua_media.c to use new 'pj_ice_strans_cfg' setting.

[nanang]

In 5340:

Re #422: Omit IPv6 link-local address in gathering ICE candidate.

[nanang]

In 5342:

Re #422: Enable IPv6 in ICE transport in pjsua-lib:

• currently only IPv6 host candidate will be generated, still not sure whether IPv6 NAT is really used in real world
• if the account enables IPv6 media and the host has IPv4 address, ICE will generate IPv4+IPv6 candidates, it should be very useful in NAT64 environment, and should not add delay in ICE nego in IPv6 only environment (note: candidate check pair must have same IP address family).