Add a lttng-ctl header to facilitate code separation

In order for lttng-ctl.c not to grow into an uncontrollable monster of
code, add this helper header and remove static declaration of some
functions to be exported out of the C file and set them hidden.

Future work on snapshot and trigger will use it in order to better split
the code into different files.

Signed-off-by: David Goulet <dgoulet@efficios.com>