Fix bugs in posix thread-id calculation when sizeof(pthread_t) != sizeof(long)

When pthread_t was smaller, our calculated thread IDs would include
uninitialized RAM, and so our unit tests would fail because thread_ids
would never match one another.

When pthread_t was larger and alignment was big-endian, our calculated
thread IDs would only have the most significant bytes of the
pthread_t, when in practice all the entropy is in the low-order bytes.

Found with help from Dagobert Michelsen.

configure.in

@@ -529,6 +529,10 @@ if test x$bwin32 != xtrue && test "$enable_thread_support" != "no"; then
 		[Define if we have pthreads on this system])
 	have_pthreads=yes])
   CFLAGS="$CFLAGS $PTHREAD_CFLAGS"
+  AC_CHECK_SIZEOF(pthread_t, ,
+     [AC_INCLUDES_DEFAULT()
+      #include <pthread.h> ]
+  )
 fi
 AM_CONDITIONAL(PTHREADS, [test "$have_pthreads" != "no" && test "$enable_thread_support" != "no"])
 

evthread_pthread.c

@@ -33,6 +33,7 @@ struct event_base;
 #include <event2/thread.h>
 
 #include <stdlib.h>
+#include <string.h>
 #include "mm-internal.h"
 #include "evthread-internal.h"
 
@@ -84,10 +85,17 @@ evthread_posix_get_id(void)
 {
 	union {
 		pthread_t thr;
+#if _EVENT_SIZEOF_PTHREAD_T > _EVENT_SIZEOF_LONG
+		ev_uint64_t id;
+#else
 		unsigned long id;
+#endif
 	} r;
+#if _EVENT_SIZEOF_PTHREAD_T < _EVENT_SIZEOF_LONG
+	memset(&r, 0, sizeof(r));
+#endif
 	r.thr = pthread_self();
-	return r.id;
+	return (unsigned long)r.id;
 }
 
 static void *