Improve support for different proxy authentication types

[aviast]

The use_proxy() function does not include an argument to specify the type of authentication; please allow proxyauth to be specified here. The values for the type argument to authenticate() are all valid for the proxy authentication.
If a user chooses to configure the proxy settings with the config() function, then an "integer/logical" type is expected. My preference would be to allow character values so this setting can use the same values as mentioned above; the alternative being that a variable would need to be defined for each authentication option so that the numeric values can be supplied.
When a client sends a GET request the proxy responds with 407 (Proxy authentication required) and a header Proxy-Authenticate with value NTLM. I would expect libcurl could use this to automatically handle NTML proxy servers? (According to the docco libcurl defaults to CURLAUTH_BASIC, not CURLAUTH_ANY.)