Better connection sharing by using only one curl object.

* bzrlib/transport/http/_pycurl.py:
(PyCurlTransport.__init__, PyCurlTransport.has,
PyCurlTransport._get_full): Use only one curl object.
(PyCurlTransport._get_ranged): Use only one curl object. Specify
'Range' as an additional header.
(PyCurlTransport._curl_perform): Take additional headers into
account.
(PyCurlTransport._set_curl_options): Leave _perform do the header
setting.
(PyCurlTransport._curl_perform): Set all headers just before
issuing the request.