Log when a logout is successful

[AtlasLinden]

Add a message to the viewer log when a logout is successful.

[akleshchev]

Viewer is expected to print "Logging out as agent: agent_id Session: session_id"

[AtlasLinden]

Passed QA.
Verified on Second Life Release 7.2.0.16356654553 (2025.05) on Win11/MacOS

Log appears as the following:
LLAppViewer::sendLogoutRequest : Logging out as agent: agent_id Session: session_id