hdf5_ep

[svn-r2286] Gave the threading functions return types (herr_t). pthreads' calls
return 0 on success and non-zero on failure. That's what happens with
these calls.

There was a problem compiling threading on Linux. The pthread_t type is
not consistent among different implementations, so it cannot simply be
assigned to NULL or tested against it. I initialize it by calling
HDmemset(foo_thread, 0, sizeof(pthread_t)). To see if it's a "null"
pthread, I created a special pthread_t object (assigned to only in the
init phase and then only read...i.e., thread safe) and assigned it "null"
as above. Then I use pthread_equal() to determine if the thread is null.