Implement client-specific tags

Implement client-specific tags

... which allow Privoxy admins to pre-define tags that
are set for all requests from clients that previously
opted-in through the CGI interface.

They are useful in multi-user setups where admins may
want to allow users to disable certain actions and filters
for themselves without affecting others.

In single-user setups they are useful to allow more
fine-grained toggling. For example to disable request
blocking while still crunching cookies, or to disable
experimental filters only.

This is an experimental feature, to enable it configure
with --enable-client-tags. The syntax and behaviour may
change in future versions.

Implements TODO list item #144 and #145.
Funded by a donation from Robert Klemme.