CHOP-661: add and use code to listen on next available server port.

In indra/llmessage/tests/testrunner.py, introduce new freeport() function to
try a caller-specified expression (such as instantiating an object that will
listen on a server port) with a range of candidate port numbers until the
expression produces a value instead of EADDRINUSE exception.
Change test_llsdmessage_peer.py and test_llxmlrpc_peer.py to use freeport() to
construct their server class inline BEFORE launching the thread that will run
it, then pass that server's serve_forever method to daemon thread. Also set
os.environ["PORT"] to selected environment variable before running subject
test program.
In indra/llmessage/tests/commtest.h, introduce commtest_data::getport() to
read port number from specified environment variable, throwing exception if
variable not set or non-numeric. Construct default LLHost from getport("PORT")
instead of hardcoded constant.
Change indra/newview/tests/llxmlrpclistener_test.cpp to use commtest_data::
getport("PORT") instead of hardcoded constant. Also use LLSD::with() rather
than older LLSD::insert() syntax.
HOWEVER -- I am irritated to discover that llxmlrpclistener_test IS NOT RUN or
even built by newview/CMakeLists.txt! It's not even commented out -- it's
entirely deleted! I am determined to restore this test. However, as it will
take some fiddling with new link-time dependencies, that will be a separate
commit.