Fixes and Cleanup

- Fixed build failures on Windows. The timespec struct is not supported by older versions of Visual Studio. I changed the internal representation inside of the *_timespec_timepoint classes to a boost::intmax_t representing the number of nanoseconds since the epoch.
- Fixed some functions that wouldn't execute at all if they were provided a negative time duration or an absolute time that was in the past. From what I understand, they should instead execute once and then return immediately.
- Moved pthread/timespec.hpp to detail/timespec.hpp.
- Deleted detail/internal_clock.hpp and moved the seven relevant lines into detail/timespec.hpp. This keeps all of the internal clock declarations in one place.
- Renamed thread_detail::internal_clock_t to detail::internal_chrono_clock to be consistent with and yet clearly differentiated from detail::internal_timespec_clock.
- Removed "using namespace chrono" to eliminate ambiguious namespace resolution when referencing detail::internal_chrono_clock.
- Re-enabled a few tests on Windows that had previously been disabled. I want to see whether or not they still need to be disabled.

src/pthread/thread.cpp

@@ -439,14 +439,15 @@ namespace boost
                 {
                   // Use pthread_delay_np or nanosleep whenever possible here in the no_interruption_point
                   // namespace because they do not provide an interruption point.
+                  timespec const& ts2 = ts.getTs();
     #   if defined(BOOST_HAS_PTHREAD_DELAY_NP)
     #     if defined(__IBMCPP__) ||  defined(_AIX)
-                  BOOST_VERIFY(!pthread_delay_np(const_cast<timespec*>(&ts.get())));
+                  BOOST_VERIFY(!pthread_delay_np(const_cast<timespec*>(&ts2)));
     #     else
-                  BOOST_VERIFY(!pthread_delay_np(&ts.get()));
+                  BOOST_VERIFY(!pthread_delay_np(&ts2));
     #     endif
     #   elif defined(BOOST_HAS_NANOSLEEP)
-                  nanosleep(&ts.get(), 0);
+                  nanosleep(&ts2, 0);
     #   else
                   // Fall back to using a condition variable even though it does provide an interruption point.
                   const detail::internal_timespec_timepoint& ts2 = detail::internal_timespec_clock::now() + ts;