add ntlm (MS) proxy support (wishlist)

[Chad Waters]

Binary package hint: apt

Please add NTLM (MS) proxy support to apt.

Currently the Acquire::http::Proxy parameter is incompatible with the domain\username syntax.

That proxy I used as a test was MS ISA I get the following with all attempts:
 "( The ISA Server requires authorization to fulfill the request. Access to the Web Proxy service is denied. )"

See also:
 https://lists.ubuntu.com/archives/ubuntu-us-pa/2007-September/000262.html