Version 3.0 of ebtree has been imported in haproxy 1.3.14. The files have
been split into two directories :
  - src/eb*.c
  - include/common/eb*.h

The .c files had their #include changed to find the include files in the
common subdirectory. Changes have been committed right after the merge
without the files being used. They are known to build without warnings
on Linux at this stage.

Also, some optimizations are not redefined if already known: REGPRM*
and likely/unlikely which are used in ebtree are also used and defined
in haproxy. Thus, we just conditionally define them.

Last, all eb*tree*.h have been adapted to support being included multiple
times, using #ifndef/#define/#endif.