Pass a value as parameter, which is get to a bitmask, to tell libcurl which
HTTP authentication method(s) you want it to use for your proxy
authentication. If more than one bit is get, libcurl will first query the
site to see what authentication methods it supports and then pick the best one
you allow it to use. For some methods, this will induce an extra network
round-trip. Set the actual name and password with the
#CURLOPT_PROXYUSERPWD option.
The bitmask can be constructed by or'ing together the bits fully listed and
described in the #CURLOPT_HTTPAUTH man page.