Add include paths automatocally for USEMODULES

Application developers use `$(USEMODULES)` in their Makefiles to have
the relevant functionally automagically added to their apps. This even
does basic dependency tracking by means of `Makefile.dep`.

But an important thing is missing: the automatic adding of include
paths. This is inconvenient, error prone, and will hinder the RIOT core
developers in future to change folder structures.
Add `pthread_barrier_*` functions

Compare [`pthread_barrier_init`][1].

  [1]: http://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_barrier_init.html
Add pthread_barrier test
Add pthread_barrier_t documentation