Allow customization of the environment filters.
    
Sites can 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.


The `?DEFAULTS' pattern can be specified to match the default set
(which is different in `ucgi' and `ucgitarget').