Header probe/usage cleanup; notably, no longer use <sys/queue.h>.

 * configure.ac: Move the header-file checks up above the library
   checks (since some of the library checks depend on header checks).
   Consolidate invocations of AC_CHECK_HEADERS.  Don't check for
   sys/queue.h or its contents.  Use AC_CHECK_DECLS to look for
   timeradd, timerclear, timerisset; don't check for timercmp since
   we're not using the system version anyway.
 * WIN32-Code/tree.h: delete, unused.
 * event2/util.h: Adjust for use of AC_CHECK_DECLS.

 * compat/sys/queue.h: Move to queue-internal.h.
 * event-internal.h: Remove fallback definitions of TAILQ stuff.
 * event2/rpc_struct.h: Add yet another duplicate temporary definition
   of TAILQ_ENTRY.
 * many files: include queue-internal.h instead of sys/queue.h.