Support for reliable provisional response (100rel, PRACK)

[bennylp]

Done in r1463:

• Added new files in pjsip-ua library: sip_100rel.h and sip_100rel.c
• Added PJSIP_HAS_100REL build configuration
• Added require_100rel in pjsua_config and pjsua_acc_config
• Transaction was added with a function to (re)transmit response without affecting transaction state (pjsip_tsx_retransmit_no_state)
• Invite session was updated with PRACK support.
• All build files (Makefile, WinCE, Symbian, etc.) have been updated
• See group__PJSIP__100REL.htm for the documentation

[bennylp]

See also relevant ticket #387: UAS dialog should add Contact header in 1xx response

[bennylp]

In r1467:

• fixed bug in 100rel processing that caused 2xx response to be enqueued indefinitely when the UAS previously sent 1xx response with SDP.

[bennylp]

Relevant to this, ticket #5 (support for UPDATE) has modified major parts of PRACK implementation.

PRACK now is no longer optional module since it is tightly coupled with offer/answer negotiation, hence PJSIP_HAS_100REL macro has been removed.

[bennylp]

In r1470:

• fixed bug in pjsua-lib: 100rel module is not started because PJSIP_HAS_100REL macro has been removed (above)

[bennylp]

In r1474:

• fixed compilation warnings with gcc

[bennylp]

In r1475:

• fixed compilation warnings with gcc

[bennylp]

In r1538:

• fixed bug: 100rel support was not enabled by default in PJSUA-LIB