Support for SRTP

[bennylp]

Please see the documentation in SRTP wiki page.

[bennylp]

The bulk of the work was done in separate branch, you can see the detailed changelogs for that branch in ticket #452.

Detailed changelogs on trunk is below.

r1729:

• added <stdint.h> header detection in autoconf, and store the result in PJ_HAS_STDINT_H macro in <pj/compat/os_auto.h>

r1730:

• imported libsrtp into third_party directory

r1732:

• Added PJSIP_ESESSIONINSECURE error code in sip_errno.[hc] to require that secure session shall be used

r1733:

• related to this ticket: added new invite session API pjsip_inv_verify_request2() which takes additional remote SDP, to avoid parsing SDP multiple times

r1735: (the bulk of SRTP changes, back-ported from SRTP branch)

• new files in PJMEDIA: transport_srtp.[hc]
• new configuration macro: PJMEDIA_HAS_SRTP (default=1)
• modified project dependencies to include libsrtp
• modified Samples-vc.mak, add libsrtp as third party library
• modified transport interface:
  • modified transport_udp and transport_ice to follow the new interface
  • modified pjsua_media.c to use the new media transport interface
• added new PJMEDIA error codes specific to SRTP, and map libsrtp error into PJMEDIA error code
• modified PJSUA-LIB API:
  • added use_srtp and srtp_secure_signaling settings in both pjsua_config and pjsua_acc_config
• added new --use-srtp option in pjsua