Merge commit from fork

Add `switch_stun_packet_verify_integrity()`, an HMAC-SHA1
MESSAGE-INTEGRITY verifier that is const and non-mutating: it runs
over a private copy of the pristine network-order packet, so the
caller's buffer and byte order stay untouched, and walks attributes
with its own unsigned bounded helper `stun_wire_attr_bounds()`
instead of the host-order iterator macros. A trailing
MESSAGE-INTEGRITY-SHA256 or FINGERPRINT after MESSAGE-INTEGRITY is
tolerated; any other trailing attribute is rejected.

Gate it in `handle_ice()` behind `ice->verify_integrity`: verify
before any ICE state is touched, keyed by message type (local
`ice->pass` for a request, remote `ice->rpass` for a response or
error response), and drop on failure. Keepalive indications carry no
MESSAGE-INTEGRITY and are ignored.

`ice->verify_integrity` is read from the `ice_verify_message_integrity`
channel variable in `switch_rtp_activate_ice()` and defaults off, so
receive-path behavior is unchanged unless it is enabled. Adds unit
tests in `tests/unit/switch_stun.c`.

conf/vanilla/vars.xml

@@ -426,6 +426,10 @@
   <X-PRE-PROCESS cmd="set" data="rtp_liberal_dtmf=true"/>
   <!-- Helps with WebRTC Audio -->
 
+  <!-- Require valid MESSAGE-INTEGRITY on inbound ICE (STUN) connectivity checks; unverified checks are
+       dropped. Default off. Uncomment and change Z- prefix to X- to enable. -->
+  <!--<Z-PRE-PROCESS cmd="set" data="ice_verify_message_integrity=true"/>-->
+
   <!-- Stock Video Avatars -->
   <X-PRE-PROCESS cmd="set" data="video_mute_png=$${images_dir}/default-mute.png"/>
   <X-PRE-PROCESS cmd="set" data="video_no_avatar_png=$${images_dir}/default-avatar.png"/>