Add a '--list-options' option to each command.

This is intended to be used for programmable Bash completion.

Modified from previous version:
- Changed '\0' to 0 to match current coding style.

Changes made by David Goulet:
- Check args for --list-options in lttng cli to stop auto execution of
  the session daemon.
- Adds --list-options to all help. (Hidding command is bad :P)
- Fix uninitialized ret value in enable channel

Signed-off-by: Simon Marchi <simon.marchi@polymtl.ca>
Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>