bpo-38863: Improve is_cgi() in http.server (GH-17312)

bpo-38863: Improve is_cgi() in http.server (GH-17312)

is_cgi() function of http.server library does not currently handle a
cgi script if one of the cgi_directories is located at the
sub-directory of given path. Since is_cgi() in CGIHTTPRequestHandler
class separates given path into (dir, rest) based on the first seen
'/', multi-level directories like /sub/dir/cgi-bin/hello.py is divided
into head=/sub, rest=dir/cgi-bin/hello.py then check whether '/sub'
exists in cgi_directories = [..., '/sub/dir/cgi-bin'].
This patch makes the is_cgi() keep expanding dir part to the next '/'
then checking if that expanded path exists in the cgi_directories.

Signed-off-by: Siwon Kang <kkangshawn@gmail.com>
https://bugs.python.org/issue38863