Interactive digest and basic authentication for apt-get

Binary package hint: apt

This patch adds interactive digest and basic access authentication for proxy and www to apt-get (user is requested to enter username and password in the console if the default proxy username and password specified in http_proxy or apt.conf are missing and authentication failed). The -q option makes apt-get automatically fail should it request interactive authentication.

Extra care was taken not to break other programs that use libapt-pkg, like synaptic and aptitude, so it should be OK if you recompile them with this patch applied (even though they won't be able to interactively request username and password, at least digest mode *should* work).

This patch is based on my previous patch that implemented only HTTP basic authentication (bug #409830 in
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=409830) and it was extended to include digest mode. So, most details in that bug report about patch implementation are still relevant for this one.