Installed headers relicensing and cleanup

- Move all locally-used code outside of include/lttng.
- All headers included in applications for tracing are relicensed
  to MIT license, except for lttng/core.h and
  lttng/usterr-signal-safe.h, which both purposefully only contain
  trivial declarations are static inline functions/macros of 10 lines or
  less. They are therefore OK to include into non-GPL applications
  without being considered as derivative work under the terms of the
  LGPLv2.1 license.
- Update version number support.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>