www-cgi/: Allow customization of the environment filters.
    
Sites can now configure `ucgi's environment filters, and end users can
configure `ucgitarget's filters.

By default, `ucgi' will look in `/etc/userv/ucgi.env-filter', but if
`UCGI_ENV_FILTER' is set in its environment, it will look there
instead.  The filter may contain wildcards and so on.

By default, `ucgitarget' looks in `.userv/ucgitarget.env-filter', or
`/etc/userv/ucgitarget.env-filter', if the former doesn't exist; but if
passed a `-e FILTER' option on its command line, it will look in the
file FILTER instead.  This filter may /not/ contain wildcards.

In both cases, if an explicitly named filter file can't be found then
the program fails; if the default filter files can't be found then they
fall back to built-in lists.

The reason for the asymmetry in interfaces is: it's hard to pass
command-line options to CGI scripts from webservers, but pretty easy to
set environment variables; whereas it's hard to pass environment
variables to a service program in a Userv configuration file, but easy
to pass command-line arguments.