Privatize headers

Remove most installed headers, which should be internal. The header
kcompat (kernel-style compatibility header) only makes sense within the
UST package: this should never be exported, nor be included into
instrumented applications.

Move the type-serializer header back into libust/. It is so internal it
does not even belong to include/ust/.

Standardize all the installed headers on non-kcompat style: use uint64_t
instead of u64 for types, open-code likely/unlikely.

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