Major code cleanup

Cleanup main.c of the session daemon. The main switch case to handle
client command is made more modular and clearer. Each command has a
function associated with it beginning by 'cmd_*'. Also adds channel.c/.h
and event.c/.h containing helper function to execute client commands.

The reste of the modified code is simple cleanup.

Some minor bugs were fixed also by this cleanup. All and for all, this
makes easier to maintain the session daeamon code base.

Signed-off-by: David Goulet <david.goulet@polymtl.ca>