serve files in or below the directory of argv[0]

Replaces the original functionality of
CGIHTTPServer.CGIHTTPRequestHandler that required all and only
executable scripts to be in ./cgi-bin.  With the changed functionality,
the directory tree can be arbitrarily deep, and all and only *.py files
are executed (others are assumed to be of content-type text/html).
2nd try, having messed up in git.