Fixed some typos, courtesy of codespell

Main change is the conversion from the British cancelling/-ed to the
American canceling/-ed.

src/libstrongswan/threading/thread.h

@@ -109,7 +109,7 @@ u_int thread_current_id();
 
 /**
  * Push a function onto the current thread's cleanup handler stack.
- * The callback function is called whenever the thread is cancelled, exits or
+ * The callback function is called whenever the thread is canceled, exits or
  * thread_cleanup_pop is called with TRUE as execute argument.
  *
  * @param cleanup		function called on thread exit