Merge branch 'cb/http-multi-curl-auth'

Fixes http authentication breakage when we keep multiple HTTP requests in
flight using curl-multi.

By Jeff King (3) and Clemens Buchacher (1)
* cb/http-multi-curl-auth:
  http: use newer curl options for setting credentials
  http: clean up leak in init_curl_http_auth
  fix http auth with multiple curl handles
  http auth fails with multiple curl handles