Explicitly look for python2 in configure.in

Explicitly look for python2 in configure.in

The executable "python" has traditionally been Python 2, but is
becoming more ambiguous as operating systems transition towards Python
3.  Look for "python2" in the path in preference to "python", and
check that what we found isn't Python 3.

Remove the "#!/usr/bin/python" headers at the start of Python test
scripts since we run them explicitly under python, not as executables.
Execute paste-kdcproxy.py via sys.executable in t_proxy.py so that it
doesn't need a #!/usr/bin/python header.

ticket: 8709 (new)