modules/sessions_consolekit.c: free ck_manager_proxy if module registration failed
trivial, fix some usession comments
add u_session_init() and call it from ulatencyd.c

Initializes internal hash tables and adds a one time timeout that will
check if an agent has been registered. If not, warning will be logged.

Without this if no agent is registered, hash tables stay set
to NULL and a lot glib assertions are logged.
add g_info() macro to ulatency.h

because the code logs G_LOG_LEVEL_INFO priority messages a lot and
I am lazy to write g_log(G_LOG_DOMAIN, G_LOG_LEVEL_INFO,...)
unref USession.proxy earlier - when it's removed from U_sessions list
fix u_session_init()
usession: change how sessions are being added in order to simplify agents

- introduce u_sesssion_new() to allocate USession structure
- u_session_add() should be called after all USession properties set
enhance and clarify log messages from usession and consolekit
rename modules sessions_{consolekit,logind) to bare consolekit and logind