Split session stuff from sysinfo.c into usession.c, sessions_consolekit.c and sessions_logind.c

No changes in code.
prepare for usession implementation: move/rename some files

move sessions_consolekit.c, sessions_logind.c to modules/
rename modules/test.c to modules/test_module.c
core: track real user sessions, reimplement consolekit agent as module

* u_session type renamed to USession
* sessions subsystem is implemented in usession.c
  - GList *U_session_list replaced with USession *U_sessions, which is
    pointer to the first USession instance.
  - u_sessions_*() family functions
* header for session tracking agents is usession-agent.h
* new USession structure members: is_valid, id, name, leader,
  consolekit_cookie
  - The "guint id" property is the actual session identification
    generated for every new session.

Consolekit agent was reimplemented into a module, other agents such
as logind may be easily added. Once the agent is registered to
the system, it's responsible for determining process sessions and
sessions leaders.

Currently, sessions are not exported to lua.

The code is heavily documented and is described by USession module
in doxygen generated documentation.
lua bindings: add U_PROC.usession_id and ulatency.U_SESSION_* constants

U_PROC.usession_id:
 - ulatencyd specific session ID
 - implemented by u_session_id_find_by_proc()

U_SESSION_ID enum:
 - ulatency.U_SESSION_UNKNOWN
 - ulatency.U_SESSION_INIT
 - ulatency.U_SESSION_KERNEL
 - ulatency.U_SESSION_NONE
 - ulatency.U_SESSION_USER_UNKNOWN
 - ulatency.U_SESSION_USER_FIRST
lua bindings: implement U_PROC.usession_is_active
scheduler.lua cgroups_name expansion: variables may contain '_' character now
update one-seat-desktop mappings to use usession
update destop mappings to use usession
lua_binding.c: cosmetic changes
rename USesssion.leader => USession.leader_pid

to avoid conflicts with ongoing lua U_SESSION.leader, which will
return the leader U_PROC instance.
export USession instances to lua as U_SESSION table
rename U_PROC.session to U_PROC.sid and change related u_proc properties

u_proc.fake_session => u_proc.fake_sid
u_proc.fake_session_old => u_proc.fake_sid_old

Reason: U_PROC.session will be used for USession instance.
describe U_PROC.sid and U_PROC.pgrp in doxygen docs
lua bindings: rename U_PROC.usession_{id,is_active} to U_PROC.session_*
rename U_SESSION_* constants to USESSION_*

Same convention with UPROC_* states.
export process USession instance to lua (U_PROC.session)
fix U_PROC.session_is_active documentation
ulatency.get_sessions() - return table of U_SESSION instances
lua: some U_SESSION related fixes in documentation
add trivial U_SESSION test