v4, 5 partsNew roll of proxy improvements

So here is a new roll based on last comments, changes from last roll are:

- Unify in new function set_proxy_auth() the code for
setting proxy username and password in cURL options, which is
indepently setted and not as part of the CURLOPT_PROXY as before.

- If using libcurl >= 7.19.1 use options CURLOPT_PROXYUSERNAME
and CURLOPT_PROXYPASSWORD which let special characters like '@'
or ':' to be present in those options and be correctly treated
by libcurl. If libcurl is < 7.19.1 then use CURLOPT_PROXYUSERPWD
option.

- Refactor http_request_reauth() function to permit more than two
retries, this comes from a jeff king's patch.

- Rename HTTP_REAUTH return status to HTTP_RETRY to better fit the
new semantics according to the aforementioned refactor in
http_request_reauth().

Regards,

Nelson Benitez Leon (5):
  http: try http_proxy env var when http.proxy config option is not set
  http: handle proxy proactive authentication
  http: handle proxy authentication failure (error 407)
  http: Avoid limit of retrying request only twice
  http: rename HTTP_REAUTH to HTTP_RETRY

 http.c |   56 ++++++++++++++++++++++++++++++++++++++++++++++++++------
 http.h |    2 +-
 2 files changed, 51 insertions(+), 7 deletions(-)