bash completion: Remove underscores in handler function names

bash completion: Remove underscores in handler function names

If the command name contains a dash, it is removed to derive the
function name that will handle completion. For example, add-context
becomes _lttng_cmd_addcontext. The functions with erroneous names
were not getting called.

Reported-by: Daniel U. Thibault <daniel.thibault@drdc-rddc.gc.ca>
Signed-off-by: Simon Marchi <simon.marchi@polymtl.ca>
Signed-off-by: David Goulet <dgoulet@efficios.com>