Add more tests for the xlmrpc.ServerProxy

Patch by Mads Kiilerich, Reviewed by Barry Warsaw.

According to #6358, python 3.0 has a different implementation that behaves differently.

Need to be careful with thread switching when testing the xmlrpc server.  The server thread may not have updated stats when the client thread tests them.

PIPE_BUF is not defined on Windows, and probably has no meaning there.

Anyway the subprocess module uses another way to perform non-blocking reads (with a thread)

cleanup.  If this fix works, it means that Solaris is unique among
our platforms in what happens when shutil.rmtree is called on the
current working directory (ie: it doesn't work on Solaris, but
it does everywhere else).

if the test gives useful failure info on Solaris buildbot.

(I copied this from another import test, but currently this will fail if
TESTFN ends up in /tmp...see issue 2609).

and make sure we test resetting all three execute bits.

hopefully reveal the real problem.

make sure that out-of-range values consistently raise struct.error.

reorganize the test_struct module to remove duplicated code and tests.

on Windows.

added the shutdown_request() which can perform shutdown before calling close.  This is needed for the ForkingMixIn because different close semantics are required for child and parent process.  shutdown_request(), for TCP servers, calls socket.shutdown() and then calls close_request().  Therefore, this is not an backwards incompatible change, since subclasses that continue to override close_request() continue to work.