Fix: improve and update lttng.h comments

Signed-off-by: David Goulet <dgoulet@efficios.com>
Split lttng.h into several files

This does NOT break backward compatibility since lttng.h includes every
possible ABI/API.

Fixes #78

Signed-off-by: David Goulet <dgoulet@efficios.com>
Remove deprecated API from lttng.h

Signed-off-by: David Goulet <dgoulet@efficios.com>
Fix: man page typo

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