Rename and export lib kernel consumer

The old liblttkconsumerd is renamed to liblttngkconsumerd and exported
publicly having the public header "lttng/lttng-kconsumerd.h".

To link with this library, use -llttngkconsumerd.

Every public functions now begins with the "lttng_" prefix to fit the
lttng public namespace.

Comments are changed also to fit 80 lines standard and the rest of the
git tree comments.

This complete renaming was motivated by the new tool made by Julien
Desfossez called 'lttngtop' which uses the kernel consumer library to
create a custom consumer. More works will add consumers in the lttv and
babeltrace projects.

Please see lttng.org to learn about lttngtop.

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